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IC 


8969 



Bureau of Mines Information Circular/1984 




Gold and Silver Leaching Practices 
in the United States 



By Peter G. Chamberlain and Michael G. Pojar 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8969 



Gold and Silver Leaching Practices 
in the United States 

By Peter G. Chamberlain and Michael G. Pojar 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



As the Nation's principal conservation agency, the Department of the Interior 
has responsibility for most of our nationally owned public lands and natural 
resources. This includes fostering the wisest use of our land and water re- 
sources, protecting our fish and wildlife, preserving the environmental and 
cultural values of our national parks and historical places, and providing for 
the enjoyment of life through outdoor recreation. The Department assesses 
our energy and mineral resources and works to assure that their development is 
in the best interests of all our people. The Department also has a major re- 
sponsibility for American Indian reservation communities and for people who 
live in Island Territories under U.S. administration. 






Library of Congress Cataloging in Publication Data; 



Chamberlain, Peter G., 1942- 

Gold and silver leaching practices in the United States. 

(Bureau of Mines information circular ; 8969) 

Bibliography: p. 36-38. 

Supt. of Docs, no.: I 28.27:8969. 

I. Gold mines and mining— United States, 2. Silver mines and 
mining— United States. 3. Solution mining— United States. I. Pojar, 
Michael G. II. Title. III. Series: Information circular (United States. 
Bureau of Mines) ; 8969. 



;E^J-2«57CJ?' [TN423.A51 622s [622'.3422l 83-600316 



For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 4 

Mineralogy 5 

Leaching technology 6 

Heap leaching ore preparation 6 

Dump leaching ore preparation 7 

In situ leaching ore preparation 7 

Leaching process 8 

i^each solutions 8 

Solution distribution 8 

Recovery 12 

Comparison of techniques 12 

Zinc precipitation 12 

Charcoal adsorption 14 

Leaching operations. 15 

Arizona 22 

California 23 

Colorado 23 

Idaho 25 

Montana 25 

Nevada 26 

New Mexico 28 

South Dakota 29 

Permitting regulations 29 

Federal regulations 29 

State regulations 30 

Leaching problems and research 33 

Percolation 33 

Temperature 33 

Solution loss 33 

Calcium salt scale 34 

Research on novel solution mining methods 34 

In situ leaching 34 

Leach farming 35 

Thin-layer leaching 35 

Summary 36 

References 36 

Appendix. — Gold and silver leaching blblllography 39 

ILLUSTRATIONS 

1 . Heap leaching system 2 

2 . Typical pilot heap leaching operation 3 

3. Dump leaching system 3 

4. In situ leaching systems 4 

5 . Fixed-spray solution distribution. 9 

6 . Ralnblrd sprinkler 9 

7. Bagdad wlggler 10 

8. Solution distribution by ponding 11 

9. Pregnant effluent solution collecting pond 11 



li 



ILLUSTRATIONS—Continued 



Page 



10. 
11. 
12. 
13. 



1. 

2. 
3. 
4. 
5. 
6. 



Zinc precipitation recovery system 13 

Charcoal adsorption recovery system 14 

Leaching operation locations in the United States 22 

In situ leaching test at Ajax Mine 24 

TABLES 

Gold and silver heap and dump leaching operations in the Western United 

States 16 

Location data for key operations 19 

Ore mineralization and process characteristics 20 

Leach solution treatment 21 

Environmental Protection Agency regional offices 30 

State permitting agencies 31 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


A 


ampere 


L/s 


liter per second 


A/ft2 


ampere per square foot 


L/8*cm"2 


liter per second 
per square centimeter 


Btu/h 


British thermal unit 








per hour 


m 


meter 


"C 


degree Celsius 


mi 


mile 


cm 


centimeter 


mln 


minute 


ft 


foot 


m2/kg 


square meter per kilogram 


ft2/lb 


square foot per pound 


mL/s*cm"2 


milliliter per second 
per square centimeter 


g 


gram 










TiiTn 


millimeter 


gal/min 


gallon per minute 










mol/mol 


mole per mole 


gal/min'ft"2 


gallon per minute 








per square foot 


oz 


ounce 


g/kg 


gram per kilogram 


oz/ton 


ounce per ton 


h 


hour 


Pa 


pascal 


in 


inch 


pet 


percent 


kg 


kilogram 


ton/d 


ton per day 


kg/d 


kilogram per day 


ton/yr 


ton per year 


kg/m^ 


kilogram per cubic meter 


V 


volt 


km 


kilometer 


W 


watt 


lb 


pound 


wk 


week 


lb/ft' 


pound per cubic foot 


wt pet 


weight percent 


lb/in2 


pound per square inch 


yr 


year 


lb/ton 


pound per ton 







1 



GOLD AND SILVER LEACHING PRACTICES IN THE UNITED STATES 

By Peter G, Chamberlain^ and Michael G» Pojar 



ABSTRACT 

The surge in gold and silver prices during the 1970' s attracted many 
new mining operators and has rekindled Interest among experienced ones. 
With Its low capital Investment requirements and fast payout, leaching 
has attracted many operators — particularly those with small or low- 
grade deposits. Although In certain situations leaching offers many 
advantages over conventional mining methods , many operators are uncer- 
tain how these relatively new techniques should be Implemented. Conse- 
quently the Bureau of Mines has prepared this circular to disseminate 
information on gold and silver leaching practices, techniques, and 
problems. Engineering data gathered from 26 operations indicate that 
most ores are leached in heaps following crushing and distribution on 
pads. Metal values are recovered from cyanide leach solutions using 
either zinc precipitation or charcoal adsorption. Potential problems 
that may hamper on block development of a leaching operation are poor 
percolation characteristics of the ore, calcium salt buildup, low tem- 
peratures, and solution losses. An extensive bibliography on gold and 
silver leaching is appended. 



^Supervisory mining engineer. 
^Mining engineer. 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



Climbing gold and silver prices during 
the 1970' s awakened dormant interest in 
mining these metals in many districts. 
Since gold and silver deposits frequently 
can be profitably mined by small opera- 
tors, a mix of mining activity has been 
created, comprised of large and small 
companies, both experienced and unexperi- 
enced. A key factor in the profitability 
of small mining operations is the advent 
of an alternative to conventional mining 
and milling operations — solution mining 
(leaching). Basically gold and silver 
leaching involves spraying a cyanide so- 
lution on the ore to dissolve the metal 
values, collecting the solution contain- 
ing the dissolved metals , and recovering 
the metal from the solution. By elimi- 
nating milling, leaching reduces capital 
cost and startup time for new operations. 
Operating costs are likewise signif- 
icantly lower. The disadvantages of 
leaching are lower recovery and greater 



difficulty in controlling the cyanidation 
process. 

There are three types of leaching 
systems — heap, dump, and in situ; "vat" 
leaching accompanying conventional mill- 
ing operations is not considered in this 
publication. If ore is mined or if it is 
gathered from old mine waste-rock piles 
and hauled to specially prepared pads 
lined with clay, tar, or Hypalon^ for 
leaching, the method is "heap" leach- 
ing (figs, 1-2), The rock is frequently 
crushed before being placed on the pad. 

If mine waste-rock piles or dumps are 
judged to contain sufficient mineral 
value to justify leaching and the 
solutions can be controlled without 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



\ 




\ Barren solution 
from processing 
plant 



Solution 
sprays 



i^ To processing 
plont 



Pump 
FIGURE 1. - Heap leaching system. 




FIGURE 2. - Typical pilot heap leaching operation. 



J2T \ 



Waste rock 
dump 




Pregnant solution 
drainage 



Barren solution 

from processing j 1 

plant ^ A ^ 



Solution makeup 
tank 



To processing i ^ 

plant ) 



Pump 



FIGURE 3. - Dump leaching system. 



appreciable losses, the pile is "dump" 
leached without any preparation (fig. 3). 
Although this technique is rarely used 
for leaching gold and silver, it is com- 
mon in the copper industry. 

Finally if the ore is broken and left 
in place or if it conducts fluids flow 
without blasting, it can be leached "in 
situ" or in-place (fig. A). An exposed 
ore body can be leached in situ by spray- 
ing solution on the surface and collect- 
ing it in recovery wells after it has 
percolated down through the ore. For 
buried ore bodies, the solution must be 
injected into the formation through in- 
jection wells and recovered from adjacent 
recovery wells. Although copper and ura- 
nium have been leached in situ, there 
have been only sporadic attempts to de- 
velop such operations for leaching gold 
and silver. One attempt to in situ "leach 
gold at the Ajax Mine near Victor, CO, is 
described later. 

Since many operators are considering 
leaching gold and silver for the first 
time or are experiencing problems in es- 
tablishing leaching operations, this re- 
port summarizes leaching principles and 
practices, discusses problems that may be 
encountered, and lists sources of addi- 
tional information in a bibliographic 
appendix. 



Solution 
sprays 



1 



7i-rote55ing . Darren 
plant \ makeup 



Barren solution 
makeup tank 




/ / Solution 
-T^ flow 




Perforated 
casing 

Recovery 
well 



o 



EXPOSED ORE 
BODY 



Pump 




BURIED ORE 
BODY 



Pump 



FIGURE 4. - In situ leaching systems. 



ACKNOWLEDGMENTS 



The authors wish to express apprecia- 
tion to the following mining companies 



that provided engineering data on their 
leaching experiments or operations: 



Company 



Location 



American Selco, Inc Reno, NV 

Can-American Mining Co Tombstone, AZ 

Carlin Gold Mining Co Carlin, NV 

Congress Consolidated Coal Mining Co. Phoenix, AZ 

Cyprus Exploration Co Carson City, NV 

D Z Exploration Co Lovelock , NV 

Gold Creek Corp Eureka and Ely , NV 

Gold Resources Joint Venture Cripple Creek, CO 

Golden Arrow, Inc Las Vegas, NV 

Hildebrand Drilling Phoenix, AZ 

Landusky Mining Co Landusky , MT 



Company — Con , 



Loca t ion — Con . 



Occidental Minerals Corp Hawthorne , NV 

Placer Amex Inc San Francisco , CA 

Scholz Minerals Engineering, Inc Helena, MT 

Silver Ridge Mining Co Tombstone , AZ 

Smoky Valley Mining Co Round Mountain, NV 

State of Maine Mining Co Tombstone , AZ 

Tombstone Exploration, Inc Tombstone, AZ 

Vekol Mine Development Co Chandler , AZ 

Windfall Venture Eureka, NV 

Zor tman Mining Co Mica, WA 

MINERALOGY 



Since many good references are avail- 
able on the geology of gold deposits, 
such information is not provided herein. 
Of particular importance however, in 
evaluating the leachability of gold- 
silver deposits is their mineralogy. 

Gold is usually deposited as native or 
free gold associated with pyrite (_5).'* 
Occasionally, as at Cripple Creek, CO, 
the gold is deposited as a telluride. 
Various heavy metal compounds are fre- 
quently associated with the gold. 

Silver is usually deposited in its 
compound form. Besides native silver 
(Ag), those minerals containing leach- 
able silver are argentite (silver sul- 
fide, Ag2S), cerargyrite (silver chlo- 
ride, AgCl), embolite (AgCl, AgBr), and 
bromyrite (silver bromide, AgBr). Other 
silver minerals are not readily leachable 
(JL2, 24). 

Ore can be economically leached at 
grades about an order of magnitude lower 
than they are commonly milled. Current 
leaching operations are producing gold 
from ores containing as little as 0.03 
oz/ton with cutoff grades down to 0.01 
oz/ton. Most silver leaching operations 
produce from ores grading 1 to 4 oz/ton. 
The easiest ores to leach are those 
that have been weathered or oxidized, 
liberating the gold or silver from pyrite 
or other encapsulating minerals. 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



A variety of mineralogical conditions 
can hamper or prevent leaching of an 
ore. For example, deposits that contain 
organic carbon are not suitable because 
the carbon prevents much of the gold from 
dissolving and adsorbs any dissolved met- 
al before the leach solutions are recov- 
ered. Other refractory ores are those in 
which the gold or silver is totally en- 
cased by an impervious matrix material 
such as quartz so that the leaching solu- 
tions cannot contact the metal. Copper, 
cobalt, and zinc in the ore may preferen- 
tially take the place of gold and silver 
in the leaching reaction and greatly 
reduce the reaction with the desired 
metal. 

Some ores, such as tellurides or those 
containing arsenopyrite or antimony, must 
be roasted before they can be leached 
with cyanide and so are not amenable 
to heap leaching (12) . Although other 
leaching solutions have been investigated 
for use with telluride deposits, no com- 
mercial heap leaching operations have 
resulted. 

Pyrrhotite is another mineral that com- 
plicates leaching. Decomposition of pyr- 
rhotite in cyanide produces ferrocyanide, 
which removes free cyanide from solution 
and prevents its reaction with the gold 
or silver. This decomposition also re- 
moves oxygen from the solution, which 
further decreases the reaction with gold 
and silver. 

If manganese occurs with silver ores, 
its higher order oxidation products can 



form refractory compounds of silver and 
manganese (8^, 38) . Many silver deposits 
were left unmined throughout the Western 
United States because of this particu- 
lar problem. Some of these deposits may 
be amenable to dual leaching, first 
with aqueous SO2 to recover manganese, 
followed by neutralization and leaching 
with cyanide to recover silver. 



Clay minerals in the ore pose a major 
problem in leaching operations. The clay 
particles block the leaching solution's 
flow through the ore and isolate large 
portions of ore from the solution. Some 
heaps contain so much clay that the solu- 
tion perches on top with negligible down- 
ward percolation. 



LEACHING TECHNOLOGY 



HEAP LEACHING ORE PREPARATION 

Ore preparation for heap leaching con- 
sists of (1) preparing an impervious pad, 

(2) mining or gathering ore from dumps, 

(3) crushing the ore (optional), and 

(4) placing the ore on the pads. 

Fads are usually situated in flat ter- 
rain that is graded to provide gently 
sloping surface for drainage (2 to 5 
pet). Next the pad site must be lined to 
prevent solution seepage losses. If 
plastic liners are used, the ground is 
covered with a layer of sand or tailing 
that is rolled to provide a cushion. The 
liner sections are laid out and cemented 
Together, then covered with sand to pro- 
vide additional cushioning. Heaps that 
are subjected to much vehicular traffic 
are often lined with asphalt. Some pads 
are merely lined with a thick clay or 
tailing layer that becomes essentially 
impervious when wet. A dam or berm with 
a drain is constructed on the downstream 
end of the pad to direct the solution 
into a holding pond. 

Size distribution of ore placed on 
heaps ranges from run-of-mine to minus 
1/4 in (6 mm). Many miners crush the ore 
to significantly improve total recovery 
and recovery rates. Operating costs are, 
of course, higher when ore is crushed. 
Before an operator selects a particular 
particle size, the ore should be tested 
to determine the trade-off between miner- 
al recovery and crushing costs at several 
sizes. 

Ore is spread on the prepared pads by 
scrapers, front-end loaders, trucks, and 
bulldozers; conveyors and stackers can 
also be used. A unique gantry system has 
been employed at one operation in New 



Mexico ( 22 ) , Since solution percolation 
into a heap is severely affected by any 
packing from driving on the surface of 
the heap, the specific technique by which 
the ore is placed in heaps profoundly in- 
fluences precious metal recovery. Tech- 
niques that eliminate traffic on the im- 
placed ore are strongly urged. 

Each layer, called a lift, is usually 
placed 5 to 10 ft (2 to 3 m) deep; the 
range encountered at current operations 
was 2 ft (1 m) to 20 ft (7 m) . Although 
it has long been felt that after solu- 
tions percolate through a heap greater 
than about 10 ft (3 m) deep, they may be- 
come oxygen deficient, the rate of oxygen 
depletion has not been accurately mea- 
sured. Recent experiments with hydrogen 
peroxide additives have shown no in- 
creased metal recovery (40) , which would 
indicate that the critical oxygen content 
is not as high as originally envisioned. 

After the first lift has been leached, 
either it is removed from the pad, or the 
next lift is placed on top of it so that 
subsequent applications of leaching solu- 
tions will percolate through both, A 
third and fourth lift can be added later. 
Operations using fine ore size generally 
leach with only one lift because at the 
end of the leaching cycle the ore is 
virtually depleted and may as well be 
removed. Operations with larger sized 
ore particles that take longer to leach 
generally use multiple lifts. The solu- 
tion can then pick up extra mineral val- 
ues from the lower lifts until they are 
depleted without wasting valuable pad 
space. 

Placing ore on heaps so as to prevent 
surface packing is a problem. If vehicu- 
lar traffic packs the surface into a hard 



pan, the leaching solution will not per- 
colate uniformly downward; in fact, stag- 
nant puddles or ponding on the surface 
may occur. Although packed material can 
be loosened with a ripper, several compa- 
nies are considering conveyor systems to 
eliminate vehicular traffic on heaps. 
Pushing the material into place with a 
front end loader after it is dumped on 
the pad also minimizes surface packing. 

Where clay in the ore causes percola- 
tion problems, a relatively new technique 
of agglomerating the fines can be used 
to prepare the ore. Devised by the Bu- 
reau of Mines' Reno Research Center, the 
technique dramatically improved percola- 
tion rates, prevented channeling, and im- 
proved leaching rates in pilot tests (26- 
28 ) . Several companies have used the 
technique in full-scale operations since 
1980. At one of these sites, recovery 
improved from 37 to 90 pet as a result 
of agglomeration. The technique involves 
mixing the dry crushed ore with 5 to 15 
lb/ ton (2.5 to 7.5 g/kg) port land cement, 
wetting with 8 to 16 wt pet water, me- 
chanically tumbling the wetted feed to 
effect agglomeration, and curing the ag- 
glomerated material for 72 h prior to 
leaching. The lime in the port land ce- 
ment reduces the amount of lime that must 
be added to the leaching solution to 
maintain the proper pH; see the following 
section on leaching solutions. Even 
greater benefits accrue if, instead of 
using water, the cement-ore mixture is 
wetted with relatively strong cyanide so- 
lution during agglomeration. This cya- 
nide solution can then begin precious 
metal dissolution while the pellets are 
curing so that the heap can be leached 
with water that can be recirculated 
throughout the leaching cycle. Prelimi- 
nary tests resulted in reduced cyanide 
consumptions and reduced leaching time. 

DUMP LEACHING ORE PREPARATION 

Although waste rock from either under- 
ground or open pit mines is of too low 
a grade to warrant conventional mill- 
ing, some gold or silver may be recovered 
by leaching it. If dumps are leached 



without additional ore preparation or 
transporting the rock to prepared pads, 
the operation is termed a dump leach. 
Caution must be exercised in selecting 
dumps to insure that no leach solutions 
can escape into ground water or surface 
water drainage. A heavy clay soil under- 
lying the dump is necessary to prevent 
solutions from percolating to the ground 
water. Dams are generally constructed to 
trap and hold the leach solutions. 

IN SITU LEACHING ORE PREPARATION 

There are no commercial-scale in situ 
gold or silver leaching operations in the 
United States. If such leaching is un- 
dertaken, ore will probably be prepared 
by blasting the formation to rubblize it. 
For shallow deposits [less than 300 ft 
(100 m)] the deposits would be rubblized 
with explosives placed in vertical holes 
patterned after techniques used for in- 
place copper leaching (9-10) . Deep de- 
posits would require blasting methods 
similar to those employed at conventional 
underground mines. For instance, if a 
deposit would be developed by a certain 
stoping configuration for conventional 
mining, the same configuration would 
likely be the most efficient for prepar- 
ing the ore for in situ leaching. The 
only difference would be that in conven- 
tional operations all of the rock from 
development openings and ore from the 
stopes is hauled to the surface, whereas 
in leaching operations only the 20 to 25 
pet of this material would be hauled to 
the surface to provide "swell" space for 
the rubblization blast. 

Certain shallow underground mines in 
well-fractured formations may be amenable 
to in situ leaching without any ore prep- 
aration. Access to the deposit would be 
gained via the mine development openings; 
solution injection and recovery wells 
could be drilled from any of these open- 
ings at possible cost savings (by eli- 
minating drilling through barren over- 
burden) . Placer gold deposits would 
probably be permeable enough to permit 
leaching from vertical wells without 
blasting (33). 



LEACHING PROCESS 

Leach Solutions 

Although it is possible to leach gold 
and silver with several types of solu- 
tions, all current operations use sodi- 
um cyanide (NaCN) , mixed in water at 
strengths of about I lb/ton (0.5 g/kg) of 
solution, or 0.05 pet. Strengths encoun- 
tered in current operations range from 
0.3 to 5.0 lb/ton (0.15 to 2.5 g/kg). 
The higher strengths are generally used 
on ores with high silver content. 

When the cyanide solution contacts free 
gold or silver, leaching occurs according 
to the following reactions (2^, 12) ; 

2Au + ANaCN + O2 + 2H2O 

->■ 2NaAu(CN)2 + H2O2 + 2NaOH 

and 4Au + 8NaCN + O2 + 2H2O 

-»■ 4NaAu(CN)2 + 4NaOH. 

The reaction depends strongly on oxygen, 
which is added by bubbling air through 
the solution and/or by spraying the solu- 
tion onto the heaps. 

When other silver minerals are leached, 
the silver similarly combines with the 
cyanide ion except that silver chloride 
apparently will combine without oxygen 
(24) ; for instance, 

AgCl + 2NaCN ->■ NaAg(CN)2 + NaCl. 

Leach solutions are effective at pH 9.5 
to 11, although both lower and higher al- 
kalinities have been successfully used. 
Lower pH may result in decomposition of 
the cyanide by hydrolysis or by reaction 
with carbon dioxide in the air. Low pH 
can also permit gasification (and hence 
loss) of the cyanide if the solution con- 
tacts natural ground acids (19) . Con- 
versely, excessively high alkalinity 
seems to retard the reaction. The de- 
sired pH is maintained by adding lime 
(CaO) or caustic soda [sodium hydroxide 
(NaOH)] at about 0.5 to 1.0 lb/ton (0.25 



to 0.5 g/kg) 
tic soda is 
maintenance 
scale. Some 
the cyanide 
be higher for 



of solution. Although caus- 
more expensive, it reduces 

problems caused by lime 
operators claim that both 

concentration and pH should 
silver ores than for gold 



ores , but no trend could be discerned 
from the engineering data available. 

Solution Distribution 

Leach solutions are pumped from a mix- 
ing pond to the distribution site after 
the cyanide and lime (or caustic soda) 
have been added. Upon reaching the site, 
the solution travels through a grid of 
distribution lines (usually 3/4- to 2-in- 
diam plastic tubing) deployed across the 
top of the heap or dump. Spray nozzles, 
sprinkler heads, or wriggler tubing con- 
nected at various intervals into the main 
distribution line apply the solution. 

Fixed-spray systems are the cheapest 
and easiest to install; some operators 
merely punch holes in the distribution 
lines at fixed intervals to create a 
spray (fig. 5). Although these require 
little maintenance, channeling is a com- 
mon problem. Other operators may attach 
short feeder lines connected to fixed 
lawn-type sprinklers, but these do not 
distribute the solution as uniformly as 
do other systems and frequently result in 
channeling. 

The most common solution distribution 
system is the oscillating lawn sprinkler, 
usually referred to as the rainbird 
sprinkler (fig. 6). This sprinkler gives 
a more uniform coverage than fixed 
sprays. These sprinklers are, however, 
susceptible to calcium salt scale, which 
restricts the flow. When the scale 
severely restricts solution flow, the 
sprinklers must be removed and soaked in 
hydrochloric acid to dissolve the calcium 
salts. Some operators have experimented 
with modifying the sprinkler orifice in 
an attempt to reduce this problem and to 
distribute the solution without the ring 
of more concentrated sprinkling that 
seems common from off-the-shelf sprin- 
klers. Sprinklers are generally operated 








FIGURE 5. = Fixed=spray solution distribution. 




FIGURE 6. = Rainbird sprinkler. 



10 



with 20 to 40 lb/in2 (0.14 to 0.28 x 10^ 
Pa) line pressure, which yields a radius 
of coverage averaging 35 to 50 ft (11 to 
15 m). 

The so-called Bagdad wiggler, named for 
its development at Cyprus' Bagdad Mine in 
Arizona, has recently become popular. A 
wiggler is constructed by cutting a 9-in 
(23-cm) long segment of 1/4-in (6-mm) 
thick-walled gum rubber tubing and forc- 
ing one end over a hose connection at- 
tached to the feeder lines (fig. 7). As 
the solution passes through the tubing, 
the free end wiggles and sprays the solu- 
tion around in a circle. Wigglers are 
generally placed on 10-ft (3-m) centers. 
If the wiggler flops in a figure 8 in- 
stead of a circular path, the ends are 
sliced with three short spiral cuts. 
Proponents argue that wigglers have bet- 
ter distribution patterns than sprin- 
klers. Maintenance is relatively easy; 
when calcium salt scale appears in the 
lines, the wiggler can be stretched and 
shaped to dislodge the buildup. Wigglers 
are operated at around 20- to 40-lb/in2 
(0.14 to 0.28 X 10^ Pa) line pressure. 



An even newer but similar solution dis- 
tribution technique employs the use of a 
wobbler. Wobblers are made by attaching 
lengths of thin plastic (Tygon) tubing to 
the solution feeder lines. At normal 
operating pressures the wobblers will 
produce a 4-ft (1.2-m) high spray with a 
radius of 9 ft (3 m). 

At least two operators are applying 
solutions by ponding (fig. 8). Leach so- 
lutions are directed over the top of the 
heaps, where berms and dams hold the so- 
lution in a pond. The pond is kept sev- 
eral inches deep as the solution perco- 
lates downward. A major disadvantage is 
that appreciable clay-size particles in 
the heap or dump material will thwart 
percolation and will promote channeling 
drastically. Another problem with pond- 
ing is maintaining sufficient oxygen in 
the solution for efficient dissolution of 
gold and/or silver. 

Regardless of the distribution method, 
after percolating through the ore the so- 
lution drains off the pad into a holding 
pond (fig. 9). The pond is lined with 




FIGURE 7. - Bagdad wiggler. 



u 








FIGURE 8. - Solution distribution by ponding. 



-^~ 



/%i, >fit^' 



3"" "i, r™" ?^ ■ ■»gM^iii»iiii i )t'ii'wyW i i i > > <i»iiyiy;«yr - yw i iir i rnuiiiji' 



^i 







■.^Ti 




FIGURE 9. - Pregnant effluent solution collecting pond. 



12 



plastic sheet or a clay layer to prevent 
seepage. The pond functions both as a 
surge pond and as a settling basin for 
particles contained in the pregnant solu- 
tion. Settling rates may be increased by 
adding a flocculant. 

Solution application rates vary among 
operators. Generally the rate should be 
kept below that which would cause the so- 
lution to pond on the surface and then 
channel through the ore heap or dump. 
Experiments at several sites have indi- 
cated optimum leaching rates of around 
0.005 gal/min'ft-2 (3 x lO"'* mL/s»cm-2) 
of pad area; rates at commercial opera- 
tions range from that figure up to 10 or 
20 times higher. Recovery plants have 
been established to handle throughput 
rates of 50 to 3,000 gal/min (3 to 18 
L/s). 

RECOVERY 

Comparison of Techniques 

Gold and silver are recovered from the 
pregnant solutions by precipitation with 
zinc dust or by adsorption on activated 
carbon in the form of charcoal. Each 
method had advantages and disadvantages. 
Selection of a specific system will de- 
pend on the specific conditions at a 
leaching operation and the facilities 
already available. 

Most of the conventional mill circuits 
were based on the Merrill-Crowe system 
for precipitation of gold and silver by 
zinc dust. Although the capital expendi- 
ture required to establish such a system 
has been high, there are now on the mar- 
ket, small packaged plants designed and 
priced for small leaching operations. 
Costs are reduced in these systems by 
eliminating the high-cost, countercurrent 
decantation circuit used in a conven- 
tional milling circuit. 

Where an old mill circuit can be con- 
verted to handle a leaching system, zinc 
precipitation usually costs less than 
charcoal adsorption. The low cost of 
zinc also favors zinc precipitation 



systems. One hundred pounds of zinc dust 
costs about $35 and will precipitate 600 
to 800 oz of gold, while the amount of 
activated carbon needed to adsorb about 
the same amount of gold (about 1 ton) 
costs $2,500. Although the zinc is ex- 
pended, 70 cycles must be achieved with 
the charcoal to balance the material 
costs. Zinc precipitation also does not 
require the capital and operating costs 
of a charcoal-stripping plant and kiln 
for reactivating the charcoal. 

If more silver than gold occurs in the 
leach solution, zinc precipitation offers 
a further advantage because with charcoal 
adsorption, a large volume of high-cost 
material is tied up adsorbing a lower 
value metal. The problem of recovering 
silver in leach solutions has been some- 
what alleviated by recent Bureau research 
(21) , which has demonstrated that silver 
can be effectively precipitated from the 
leaching circuit as Ag2S by adding sodium 
sulfide; the solution can then be passed 
through the carbon adsorption columns to 
recover the gold. 

Carbon adsorption systems are, however, 
becoming increasingly popular, and they 
do have several advantages over zinc pre- 
cipitation systems. The primary advan- 
tages of carbon adsorption are that the 
systems can adsorb metals from solutions 
that contain suspended solids and from 
solutions that contain low metal concen- 
trations (less than 0.05 oz/ton of solu- 
tion) (20-21). Charcoal adsorption also 
does not require a filtration system nor 
a de-aeration tank and pump. Environmen- 
tal damage from zinc salts is avoided. 

Zinc Precipitation 

A generalized flow diagram for precip- 
itating gold and/or silver with the zinc 
system is shown in figure 10. First, 
pregnant solutions are pumped from the 
holding pond to the processing plant. 
Since suspended particles will coat the 
zinc and retard its reaction, the first 
step of the process is to filter the 
solution (usually with plate and frame 
filters). The filters are often coated 



I 



Pregnant solution 
pond 



To heap •♦■ 



NaCN 



Barren 
solution 
nnakeup 
tank 



Vacuum 
pump 



Plate and 
frame filter 



De- aeration 
tower 



Pb acetate or 
Pb nitrate feeder 



Zn dust 
feeder 





Smelting 
furnace 



13 



CaO or NaOH 



Plate and 
frame filter 



I 

I Au, Ag 

.J precipitate 



FIGURE 10. - Zinc precipitation recovery system. 



with diatomaceous earth to help remove 
the suspended particles and prolong fil- 
ter life. 

Gold and/or silver precipitation also 
depends on complete removal of dis- 
solved oxygen. Although this is usually 
achieved by a Crowe-type vacuum tank and 
pump, air can also be removed by sand 
filters and ceramic baffles. Failure to 
remove the oxygen permits the precipi- 
tated gold and silver to be redissolved. 
Even more importantly, any oxygen present 
when zinc dust is added forms hydrated 
zinc oxide, which coats the zinc and pre- 
vents further reaction with the solution. 

Gold and silver precipitation are im- 
proved by adding lead acetate or lead 
nitrate to the solution. The lead forms 
a bond with the zinc that has a greater 
galvanic activity than the zinc alone and 
precipitates the precious metals faster. 
The lead acetate or nitrate added is 
about 10 wt pet of the zinc dust. Care 
must be taken to prevent coating the zinc 
with too much lead, which retards the 



galvanic action. Silver in the solution 
can form the same galvanic couple as does 
lead with zinc, so that adding lead ace- 
tate is often unnecessary for leaching 
solutions containing appreciable silver. 

Zinc dust is fed into the solution by a 
screw-type feeder and cone arrangement. 
Precipitation proceeds according to the 
following reaction (12) for gold and 
silver: 

NaAu(CN)2 + 2NaCN + Zn + H2O 

— »- Na2Zn(CN)4 + Au + H + NaOH. 

Since the zinc should precipitate gold or 
silver 1 mol/mol, approximately, equiva- 
lent amounts of zinc should be added as 
there are precious metals in the solu- 
tion. In actual practice, anywhere from 
10 to 100 pet more zinc is added than 
theoretically should be needed. Enough 
free cyanide must be present to dissolve 
the zinc so that it can replace the gold 
in the alkaline compound and so that the 
resulting zinc alkaline compound remains 



14 



in solution. The accompanying liberation 
of hydrogen is necessary to create the 
reducing conditions for precipitation. 

After the zinc has been added to and 
mixed with the leach solution, the solu- 
tion is forced through Merrill-type pres- 
sure filters. This leaves behind the 
gold and/or silver plus impurities. The 
precipitate is removed from the filter 
and dried for smelting. 

For either all-gold or all-silver pre- 
cipitates, a flux is added and the pre- 
cipitate is smelted for pouring. If both 
gold and silver are present in signif- 
icant amounts, the precipitate can be 
fluxed and smelted for sale as a dore 
bullion. If separating the gold and sil- 
ver is desired, silver can be dissolved 
from the filtered precipitate with nitric 
acid and electrowon from the solution. 
The remaining precipitate is smelted for 
obtaining its gold. As an alternative, 
gold can be separated from the precipi- 
tate by dissolving it in aqua regia and 
then recovered by precipitating it from 
the aqua regia with oxalic acid or by 
electrowinning it. 

Charcoal Adsorption 

Charcoal adsorption systems for recov- 
ering gold and silver (fig. 11) have been 
described in great detail (21). First, 
the pregnant leaching solution is pumped 
from the holding pond to the processing 
plant, where it flows through three to 
five carbon columns. Each column con- 
tains granular activated carbon manufac- 
tured from coconut shells (minus 6 plus 
16 mesh or minus 12 plus 30 mesh). The 
pregnant solution can be percolated down 
through a fixed bed of the charcoal in 
each column, or it can be directed upward 
through the charcoal at a rate that main- 
tains the charcoal in a suspended state 
(fluidized). Although columns with fixed 
beds require less charcoal for the same 
amount of solution, they are more suscep- 
tible to plugging from particles carried 
in the solution than are those with 
fluidized beds. Most commercial opera- 
tions, therefore, use fluidized beds. 
Flow rates required to maintain a 



■♦ NqCN solution ■ 



Solution 

storage 

ond 

makeup 

tank 



Barren 
solution 



','^'1*> /'/*^i ^/"^^^ 

Ore heap 



C regeneration 




Au- bearing 
solution 



Stripped C 



Au-loaded C 



L 




Recycled 
stripping 
solution 



Electrolytic cell 



Au to refinery 

FIGURE 11.- Charcoal adsorption recovery 
system (after Potter (36)). 

fluidized condition range from 15 to 25 
gal/min*ft~2 (1 to 1.7 L/s*cm~2), depend- 
ing on the size of activated carbon used. 

The columns of charcoal are arranged in 
countercurrent series so that the fresh 
solution first enters the column that 
contains the charcoal with the most ad- 
sorbed precious metals. As the solution 
flows through the charcoal, gold and sil- 
ver are adsorbed onto its surface. The 
solution passes through columns contain- 
ing charcoal with successively less ad- 
sorbed metals until it emerges as a bar- 
ren solution from the last one. After 
emerging from the last column, the barren 
solution is pumped to the makeup pond. 

When the front column of carbon (or a 
portion of it) reaches its desired load- 
ing capacity, the carbon is removed for 
stripping. An identical amount of carbon 
is then removed forward in each column. 



15 



and a fresh charge is added to the last. 
Charcoal loading formerly ranged from 400 
to 800 oz of metal per ton of charcoal 
(14 to 27 g/kg) ; a recent trend towards 
lower loading and more frequent stripping 
commonly results in loading levels of 150 
to 250 oz/ton carbon (5 to 9 g/kg). Fac- 
tors that affect charcoal loading are so- 
lution grade, flow rate, gold-to-silver 
ratio, solution pH, charcoal type, and 
impurity concentrations. 

The loaded charcoal removed from the 
columns must be stripped of the precious 
metals. Stripping is accomplished with a 
hot caustic soda solution. The tradi- 
tional Zadra process involves soaking the 
loaded charcoal at 93° C in a 1.0 pet 
NaOH-0.1 pet NaCN solution for 24 to 48 h 
(42-43). By adding 20 pet ethanol or 
methanol to the solution, the time can be 
reduced to 5 or 6 h (20) . The stripping 
time and chemical consumption can be fur- 
ther reduced by pressure stripping (37) 
with a 0.4-pct NaOH solution (without 
NaCN) at 150° to 200° C and 50 to 90 lb/ 
in (0.3 to 0.6 10 Pa). 

Gold or silver are next removed from 
the stripping solution by electrowinning. 
Typical electrowinning cells contain a 
stainless steel anode plus a stainless 
steel wool cathode for plating the metal. 



The steel wool cathode is usually packed 
to a density of 1 lb/ft (16 kg/m ) and 
provides a cathode surface area of 10 
ft /lb (2 m /kg). Operating voltages run 
from 2.5 to 3.5 V. Significantly higher 
voltages can break down the solution and 
generate hydrogen or ammonia gas, which 
blocks the plating action. Current 
ranges from 20 to 30 A, which provides a 
current density of 3 to 3.5 A/ft at the 
cathode. The solution should be retained 
in the cell at least 15 min, preferably 
30, to win the gold and silver. 

The stripped carbon is regenerated by 
heating in a kiln or chamber at approxi- 
mately 700° C (1,300° F) in a reduc- 
ing atmosphere such as steam. Although 
carbon can be used two or three times 
without regeneration, it is simpler to 
regenerate it each time rather than try- 
ing to keep track of which batch needs 
regenerating. The life expectancy of 
carbon has not been well documented. 
Some operators have experienced a 25-pct 
reduction in adsorptive capacity after 
eight or nine cycles, while others have 
found only a 33-pct reduction after 8 or 
9 yr of continuous use. A few operators 
simply sell the gold- and/or silver-laden 
charcoal for smelting rather than strip- 
ping it. That charcoal is, of course, 
destroyed during the process. 



LEACHING OPERATIONS 



Over 80 operations have been identified 
that have experimented with leaching, 
were actively leaching, or were seriously 
planning on leaching (fig. 12). Although 
many of these reported activities could 
not be verified, table 1 presents avail- 
able information on their status along 
with a general location, mine name, and 
operating company. The many new permits 
being granted each year indicate the 
great interest in leaching but make it 
impossible to generate a totally up-to- 
date list of operations. 

The 26 operations for which the most 
data were available were selected for 



presentation of geologic and operational 
parameters. Table 2 provides the loca- 
tion information for each, table 3 the 
ore characteristics and leaching data, 
and table 4 the extraction data. While 
the data contained in these tables will 
change, it is felt that operators or po- 
tential operators can use them to develop 
a feel for the range of conditions that 
can be expected at a particular site. 
The tables can also indicate possible 
solutions to site-specific problems. A 
State-by-State summary of leaching opera- 
tions follows. 



16 



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22 




FIGURE 12. - Leaching operation locations in 
the United States. 

ARIZONA 

Arizona has been second only to Nevada 
in hosting solution mining operations. 
The latest figures show that 14 opera- 
tions had been strongly considering heap 
leaching (table 1), and 6 of these have 
provided enough data to warrant inclusion 
in tables 2 to 4. 

A heap leaching operation has been es- 
tablished near the Congress Mine shafts, 
which originally supported underground 
gold mining. Rock from the waste-rock 
dumps is being crushed and hauled to the 
leaching pad, where three separate heaps 
are being leached. After rock from the 
original waste piles is exhausted, the 
company is considering opening another 
underground mine adjacent to the existing 
shafts to provide ore for leaching. 



original operations. Plans are being 
made to resume underground mining to ob- 
tain new ore when the dump material is 
exhausted. The material will probably be 
crushed to boost the recovery. The State 
of Maine Mining Co. also manufacturers 
small zinc-precipitation processing 
plants for leaching operations. 

At the nearby Contention Mine (also 
including the Grand Central and Flora 
Morrison Mines), Tombstone Exploration, 
Inc. , is removing overburden and mining 
low-grade ore over the original under- 
ground working (16) . Most of the rock 
can be mined by scrapers after it is 
ripped with bulldozers. The rock is 
crushed and presoaked with cyanide solu- 
tion before being agglomerated into low- 
strength pellets. Pellets are formed by 
a unique inclined conveyor designed by 
the company. The material is then placed 
on a pad for leaching in 6,000-ton heaps 
(5 X 10^ kg). Geologic investigations 
have delineated ore for at least 20 yr of 
operation. 

Between the State of Maine Mine and the 
Contention Mine, Can-American Mining Co. 
is operating a leaching system for silver 
at the Dry Hills Mine. Although most of 
the leach material has been obtained from 
old mining waste rock, underground devel- 
opment is underway to provide new ore for 
the heaps. Ore is crushed to minus 3/16 
in (0.5 cm) before being loaded on the 
heaps. The recovery averages 50 pet af- 
ter 2 wk of leaching in spite of manga- 
nese contained in the ore. 

Also near Tombstone, Silver Ridge Min- 
ing Co. has conducted heap leaching tests 
on ore from the old Nicholas and Gambasi- 
no Mines. The leaching tests were not 
promising because the manganese makes the 
silver somewhat refractory, and poor so- 
lution percolation occurred in the test 
heaps. Consequently ore will be pro- 
cessed in conventional mills. 



Near Tombstone, the State of Maine Min- 
ing Co. has established an operation at 
an old underground silver mine of the 
same name (17) . Feed for the heaps is 
obtained from waste rock dumps from the 



In the late 1970' s. Minerals 71, Inc., 
trucked dump material from several sil- 
ver mines around Tombstone to its site 
for heap leaching. A million-ton (0.9 
X 10^ kg) heap was apparently leached 



23 



for 3 yr before production stopped. 
Since the operation shut down in February 
1979, engineering data are currently 
unavailable. 

Vekol Mine Development Co. has estab- 
lished a heap leaching operation adjacent 
to the Vekol Mine, an old underground 
silver mine located on the Papago Indian 
Reservation south of Casa Grande. Rock 
from mine dumps is placed on a prepared 
pad for leaching. This operation has 
been on-line since 1978. A pad with a 
capacity to hold 25,000 tons (23 x 10^ 
kg) of ore is being prepared for heap 
leaching of an exposed silver ore body 
that crops out near the adit of the orig- 
inal mine. About 3 to 5 million tons 
(2.7 to 4,5 X 10^ kg) of ore could be 
removed from this ore body by surface 
mining. 

Near Humboldt, an experimental heap 
leaching pad was established at the Lit- 
tle Jessie Mine adjacent to its waste 
rock dump. The rock was placed on the 
heap without crushing. The grade of the 
return solution never reached an accept- 
able concentration even when it was mere- 
ly recycled. Part of the problem may 
have been that the high clay content in 
the rock (due to disintegration of the 
schist) caused channeling. Another rea- 
son may have been that the pyrrhotite ex- 
hausted the cyanide. No further leaching 
activities are planned. 

East of Salome an experimental opera- 
tion was established on the Robinson 
Claims at the site of an opencut gold 
mine that had been located near old 
underground mines. Rock in and around 
the pit was reconstituted into a 4,000- 
ton (4 X 10^ kg) heap for an experiment 
where leaching was carried out over sev- 
eral months to determine leaching parame- 
ters and to test equipment. Plans are 
being made to start blasting and hauling 
ore from the opencut for a commercial 
operation. 

At the site of the old San Marcos gold 
mine, east of Wenden, a small leaching 
operation has been established. Dump 
material from the mine was placed on a 



small pad and leached. An experiment was 
carried out to determine the amenability 
of the ore to agglomeration. Results of 
the experiment look promising, and plans 
are being made to expand the work. 

Several other operations that consid- 
ered leaching but have not provided engi- 
neering data or have closed before sig- 
nificant engineering data could be gath- 
ered include the Montana Mine near Ruby, 
the King Tut Mine, the Pope Mine near 
Willow Beach, the Octave Mine near Yar- 
nell, and several mines around Oatman. 

CALIFORNIA 

Although much gold has been mined in 
California, most has been recovered from 
placers by dredging, hydraulic mining, 
sluicing, and panning. Solution mining 
is noticeable by its scarcity. Only a 
leaching operation run by Chemgold at 
the Picacho Mine north of Yuma, AZ, could 
be located. At that operation, ore is 
placed in successive lifts on one big pad 
and solution is applied by "ponding." 
Gold Fields Mining Corp. has discovered a 
possibly leachable deposit — called the 
Mesquite deposit — in Imperial County, but 
no tests have yet been run. In addition. 
Anaconda has conducted agglomeration and 
leaching tests on tailing from the Darwin 
Mill, Inyo County. 

COLORADO 

The solution mining industry is not 
so well developed in Colorado as it is 
in Arizona and Nevada; only the Gold 
Resources-Newport Minerals Inc. operation 
provided enough data for inclusion in the 
engineering tables. 

Since about 1976, the Gold Resources- 
Newport Minerals Inc. joint venture has 
operated an open pit mine-heap leaching 
complex on Globe Hill. Although many of 
the Cripple Creek deposits are telluride 
(and hence not amenable to direct cyanide 
leaching) , this mine is in an oxidated 
zone of the breccia from the Cripple 
Creek volcano. A 10,000-ton/d (9 x 10^ 
kg/d) mine supplies ore for heaps. The 
ore is blasted using 5-5/8-in (14-cm) 



24 



blastholes on 10-ft (3-m) centers and is 
leached as run-of-mine. Leaching opera- 
tions extend from May to November. 

Texasgulf, Inc., joined Golden Cycle 
Corp. in a 3-yr, $3 million exploration 
program (29) to evaluate gold mining 
potential of Golden Cycle's properties 
around Victor. One of the projects was 
to renovate and explore the underground 
Ajax Mine. The work was interesting 
because of the plan to try in situ leach- 
ing with chloride and iodine solutions 
for the telluride minerals. Since the 
mine follows a very steeply dipping vein 
structure, it was hoped that the leach 
solution could be applied through hori- 
zontal drill holes following the vein's 
strike and collected in basins prepared 
in the drift after flowing down through 
the fracture system (fig. 13). Problems 
in dissolving the telluride gold ores 
and in collecting the pregnant solutions 
caused the experiment to be abandoned in 
1979. The companies recently announced 
plans for conventional mining and milling 
ore from the Ajax Mine. 



Dreco Mines, subsidiary of Southern 
Cross, Ltd., conducted a 9,000-ton (8 
X 10^ kg) heap leaching test between Del 
Norte and Torres in the eastern San Juan 
Mountains during 1981. Permits are being 
acquired for a 1,500-ton/d (1.4 x 10^ 
kg/d) open pit mining-heap leaching oper- 
ation. Another heap leaching test was 
conducted by E & B Mining near San Luis 
during the summer and fall of 1982. It 
is not known whether the company plans a 
commercial operation. 

Exploration drilling for ore body de- 
lineation has been conducted at the Gold 
Ray Mine between Copper Mountain and Min- 
eral Hill ( 30 ) . Favorable ore for leach- 
ing has been discovered; this ore is oxi- 
dized into free gold, unlike the usual 
Cripple Creek area tellurides. A leach- 
ing plant was constructed for assembling 
at the site. 

Yellow Gold of Cripple Creek, Inc., has 
adopted plans to explore the Rittenhouse 
Mine and refurbish various drifts and 
shafts (32). Part of the plan calls for 



Packers 




Injection boreholes 




Raise 



^ 



3®: 



^m^///^ ^//^//^j^^^M;^- ^/j^f-^/-a/n^^s.//^///s//f 



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jW M_ 



Pregnant solution 



*iwmm nimit^)f/^///^imi^ 




I 
I 
I 



\ 



Injection holes 



fracture 
system 



lb^\'^vv^5>^\=^v^\vx5:^x^ 



///js^^ \ n^//^ 



^ 



Pump 



Concrete tank 

SECTION VIEW 



Leach 
solution 
makeup 
tank 




7Z^J^~^— ^S^ 



END VIEW 



FIGURE 13. - In situ leaching test at Ajax Mine. 



25 



heap leaching the material in two mine 
dumps located on the property. It is not 
known if tests have been conducted on the 
material. 

Recent news articles have publicized 
the work at Saratoga Mines' sites near 
Central City. No further information is 
available at this time. 

IDAHO 

Several operations in Idaho have con- 
ducted tests on the heap leaching poten- 
tial of lean ore in and around previous 
mining districts. Information on these 
tests and any full-scale activities re- 
sulting from them has been very scant; 
only two are discussed, 

Canadian Superior Mining, Ltd, , has 
been conducting experimental heap leach- 
ing of ores from the Stibnite area since 
1974. Although the Yellow Pine Mine was 
originally an open pit operation for an- 
timony and tungsten, gold was encountered 
at one end of the pit. Recent drilling 
has delineated two nearby ore bodies — the 
West End and the Garnet Creek — that ap- 
pear favorable for leaching. In the fall 
of 1978, Canadian Superior conducted a 
500-ton (0,45 x 10^ kg) heap test on 
crushed ore; two other tests were con- 
ducted during the summer of 1979, These 
tests led to a leaching plan whereby the 
ore would be mined from two open pits , 
placed into 45,000-ton (41 x 10^ kg) 
heaps, and leached. Production began 
late in 1982 after completion of an Envi- 
ronmental Impact Statement by the U,S. 
Forest Service, 

Canadian Superior had also obtained the 
rights to explore and develop a gold min- 
ing property owned by Thunder Mountain 
Gold, Inc, , in the Thunder Mountain dis- 
trict near McCall (31), After Canadian 
Superior terminated its agreement with 
Thunder Mountain, a new agreement was 
reached with Phillips Petroleum for de- 
veloping the property with Coeur d'Alene 
Mines Corp. as the operator. The gold 
apparently occurs at a shallow depth that 
would permit open pit mining. The opera- 
tion would probably be patterned after 



plans developed for the proposed opera- 
tion in nearby Stibnite, where ore would 
be heap-leached and recovered by charcoal 
adsorption, 

MONTANA 

Heap leaching for gold and silver is 
rapidly emerging in Montana, Of the sev- 
eral operations that have expressed in- 
terest, 2 have reached full commercial 
production, and 1 is conducting large- 
scale field tests. 

The two operations are on opposite 
sides of a peak in the Little Rocky Moun- 
tains ( 15 , 6). Landusky Mining, Inc, 
and Zortman Mining, Inc, , are both owned 
by Pegasus Gold, Ltd, One mine is a few 
miles north of Landusky near the old Au- 
gust and Gold Bug Mines, An open pit 
mine produces 24,000 ton/d (22 x 10^ 
kg/d) to supply the heaps, Blastholes 
are drilled on a 10- by 8-ft (3- by 2-m) 
pattern to depths of approximately 40 ft 
(12 m) , Tlie other mine is north of Zort- 
man near the old Ruby Gulch Mine, Here 
two open pits produce 17,000 ton/d (15 
X 10^ kg/d) of ore, Blastholes are on an 
8- by 8-ft (2- by 2-m) pattern. Together 
the two operations were estimated to con- 
tain 50 million tons of reserves at ore 
grades favorable for heap leaching and 
are annually producing 40,000 oz (1.2 
X 10^ g) of gold and 90,000 oz (2,8 x 
10^ g) of silver. 

The Golden Sunlight Property 6 miles 
east of Whitehall hosted extensive un- 
derground mining for gold through the 
early 1950' s. As part of its Heavy Met- 
als Program, the Bureau of Mines and oth- 
er agencies conducted detailed economic 
analysis of conventional mining on 
the property (_1^), The present operator, 
Placer-Amex, first experimented with 
leaching on a 26,000-ton (24 x 10^ kg) 
test heap. The heap was segregated into 
five segments (each containing a differ- 
ent size distribution of ore) that pro- 
vided solution for five separate charcoal 
adsorption units. The heap was then ex- 
panded to 40,000 tons (35 x 10^ kg) for 
a full production test. Feed for this 
effort was run-of-mine rock. High clay 



26 



content in the ore caused percolation 
troubles; the heaps had to be periodical- 
ly ripped. These troubles led to a re- 
cent decision to construct a conventional 
mill complex for processing the ore. 
Heap leaching has been dropped from imme- 
diate consideration. 

In 1978 a heap leaching test was car- 
ried out on ore from the Tourmaline Queen 
Mine east of Boulder, Approximately 
25,000 tons (23 x 10^ kg) were satisfac- 
torily leached, A 30,000-ton (27 x 10^ 
kg) heap was constructed, and caustic was 
added for a new test to be conducted when 
the snow melted in the spring of 1979, 
When this test also proved successful, 
ore was obtained from an open pit mine 
for a commercial-scale heap. Since re- 
covery from this heap was lower than an- 
ticipated, the ore was crushed and re- 
placed on the heaps for leaching in 
1981, Approximately 3 million tons (2,7 
X 10^ kg) of ore graded at 0.07 oz/ton 
(0,024 g/kg) gold were delineated from 
open pit mining, 

NEVADA 

Nevada is currently the center of heap 
leaching activity. Numerous operations 
have been or are actively considering 
leaching. The Smoky Valley, the Cortez, 
and the several Carlin operations have 
all been well documented in mining liter- 
ature. Enough data are available on 11 
of these operations to warrant their in- 
clusion in tables 2 to 4, 

The Carlin Gold Mining Co, operation 
pioneered solution mining systems tar- 
geted for low-grade, disseminated, oxi- 
dized gold ore bodies. Commercial heap 
leaching operations began in 1971, fol- 
lowing laboratory and pilot scale tests. 
About 10,000 tons (9 x 10^ kg) of low- 
grade oxidized ore were leached each 
month between April and October, Four 
leaching pads were eventually placed in 
operation. Although the pads were gen- 
erally leached about 7 days before re- 
placing the ore, solutions were applied 
until their gold grade dropped below 
0,015 oz/ton (0,005 g/kg). Although the 
ore from this mine is now processed in a 



conventional mill, portions of the ore 
body are amenable to future heap leaching 
(35) , Exploration activities have uncov- 
ered two other ore deposits near the 
mine — the Maggie Creek and Gold Quarry 
Prospects — portions of which will proba- 
bly be heap-leached. Contracts have been 
awarded to construct mining and heap 
leaching facilities at the Maggie Creek 
Deposit, and approximately 2,5 million 
tons (2.3 X 10^ kg) of ore had been 
leached at the mine by the end of 1980. 

Carlin also has been dump-leaching at 
the Bootstrap Mine north of its Carlin 
pit. The Bootstrap Mine was a surface 
operation from the late 1960's until 
1978. A sizable waste rock dump on the 
property is being leached. The dump was 
originally placed on a compacted ancient 
lakebed, which makes an impervious pad 
suitable for leach solution containment. 
The dumps are merely leveled to facili- 
tate solution distribution. 

Numerous small, underground mines 
around Round Mountain produced gold from 
the turn of the century until the late 
1930' s. Now Smoky Valley has established 
an open pit mining and heap leaching 
operation near Round Mountain ( 25 , 41 ) . 
By 1977, the operation was producing 
85,000 oz (2.6 X 10^ g) of dore bullion 
per year consisting of two-thirds of gold 
and one-third of silver. To achieve this 
rate, about 8,000 tons (7 x 10^ kg) of 
ore and 7,000 tons (6 x 10^ kg) of waste 
rock are mined per day. The ore is 
crushed, placed on a pad, and leached in 
one lift. The spent ore is then rinsed 
and moved off the pad to make room for 
the next batch of ore. Five pads are in 
continuous operation; four are being 
leached and one is being reconstituted at 
any given time. Three new pads were con- 
structed during 1980. 

Another leaching operation that is well 
documented is Placer-Amex's Cortez Mine 
(13-14) . The Cortez Mine opened as a 
surface operation for a conventional mill 
in 1969. Although the ore body was ex- 
hausted by spring of 1973, the mill con- 
tinued to process ore trucked from the 
nearby Gold Acres Mine. During mining 



27 



operations, marginal ore (below mill 
grade of 0.08 oz/ton [0.03 g/kg]) was 
stockpiled. When tests run on this mate- 
rial indicated that it might be suitable, 
a heap leaching operation began in 1971. 
In 1976 the conventional zinc precipita- 
tion mill circuit was converted to an 
activated carbon adsorption system. 

Placer-Amex also operated the nearby 
Gold Acres leaching operation. Although 
currently idle, the operation has been 
well publicized (13) . The site of an 
early mine, Gold Acres was open-pit-mined 
beginning in 1973 to feed the Cortez mill 
as a replacement for the depleted Cortez 
Mine ore. When this operation proved 
successful, a new leaching plant was con- 
structed at the Gold Acres site. It was 
one of the first, if not the first, com- 
mercial applications of the charcoal ad- 
sorption system in the country. The high 
clay content of Gold Acres ore created 
lower percolation rates and recoveries 
than were obtained from the nearby Cortez 
Mine. The clayey Gold Acres ore was not 
suitable for leaching from lifts placed 
on top of each other, and periodic rip- 
ping was needed to prevent ponding. Fur- 
thermore the ore was highly carbonaceous 
and thus very refractive below the oxi- 
dized zone. Although approximately 5 
million tons (4,5 x 10^ kg) of ore had 
been heap-leached at Gold Acres and Cor- 
tez by the summer of 1977, the only cur- 
rent plans center around leaching waste 
dump rock from the idle Gold Acres prop- 
erty, Cortez has also announced plans to 
develop the nearby Horse Canyon Deposit, 
but it is anticipated that conventional 
milling rather than heap leaching will be 
used. 

South of Eureka, a Vieap leaching oper- 
ation has been in production several 
years. Near the original Windfall Mine 
underground workings, Idaho Mining Co, 
has produced ore from an open pit mine to 
feed its heap leaching operation. The 
operation is one of two that uses a pond- 
ing technique for solution distribution 
on the pads (fig. 8). Ore is hauled from 
the mine to the pad area and stacked. 
Berms are laid out on the surface of the 
pad to control the solution flow. Since 



the ore is a sandy dolomite, essentially 
clay free, percolation rates remain ac- 
ceptable. Idaho Mining also opened a new 
pit on the property in a zone that con- 
tains some clay. It will be interesting 
to see if adequate percolation can be 
maintained on ore from the new pit. In 
January 1980, Windfall Venture purchased 
the operation from Idaho Mining. 

Occidental Minerals Corp. dedicated its 
Candelaria operation in November 1980. 
At one time many underground silver mines 
were operating in the Candelaria dis- 
trict. Two of these mines — Lucky Hill 
and Mt. Diablo — are being stripped and 
open-pit mined to provide feed for a 
large heap leaching operation. Although 
the overall ore grade is good, the manga- 
nese oxide content makes it somewhat re- 
fractory, A number of experiments were 
carried out at the site, ranging from 
500-lb (200-kg) "barrel" tests up to a 
pilot heap leach on 11,000 tons (10 x 10^ 
kg) of ore. Based on the successful pi- 
lot test, plans were made to mine the two 
deposits at a rate of 8,000 tons (7 x 10^ 
kg) per day along with 16,000 tons (14 
X 10^ kg) per day waste rock. Stripping 
began in the spring of 1980, and the 
first silver was poured in the fall of 
1980. In 1981, its first year of produc- 
tion, the operation produced 1.7 million 
oz (53 X 10^ g) of silver and 9,200 oz 
(0.28 X 10^ g) of gold. Tlie company pre- 
ferred not to disclose engineering data 
from this stage of development. In June 
1982, the operation closed because of the 
decline in silver prices. In 1983 Nerco 
Metals bought Occidental Minerals and re- 
opened the Candelaria operation, 

American-Selco and Occidental Minerals 
combined on a successful heap leaching 
experiment on Alligator Ridge about 60 mi 
(96 km) northwest of Ely. First a 5,000- 
ton (4.5 X 10^ kg) heap containing 0,18 
oz/ton (0,06 g/kg) gold was leached for 
2 months, with an 80-pct recovery. Next 
the same amount of ore was leached after 
the fines were agglomerated, follow- 
ing the Bureau's technique (see section 
on ore preparation) , and the same re- 
covery was obtained in only half a 
month. Following these successful tests. 



28 



construction began in the spring of 1980, 
and the operation is now producing about 
60,000 oz (180,000 g) of gold per year. 

D Z Exploration has been conducting ex- 
periments at the Packard Mine in an old 
silver mining district northeast of Love- 
lock for two seasons. The first experi- 
ments were on run-of -mine-sized material 
from the waste dumps. Since this materi- 
al had weathered, enough fines were cre- 
ated to restrict percolation through the 
reconstituted heaps. Attention focused 
in 1979 on the Bureau's agglomeration 
technique as a means to improve percola- 
tion. To test the technique on this 
rock, ore was crushed to three sizes — 2 
in (5 cm), 7/8 in (2 cm), and 9/16 in 
(1.4 cm) — and agglomerated using 10 lb 
(4.5 kg) of cement per ton (90 kg) of 
ore. Cement was added to the crushed ore 
by a unique auger arrangement. Water 
spray was directed at the mixture, which 
then tumbled down an incline to complete 
the agglomeration. An 8-ft (2-m) high 
heap containing several hundred tons of 
each size material was then leached. 
Based on the results and additional 
drilling, full-scale production was ex- 
pected to begin in 1981, but no updated 
information is available, 

Pinson Mining Co. opened a major open 
pit mining and milling complex in Hum- 
boldt County during 1981 (39-40). Ap- 
proximately 500,000 tons (0.45 x 10^ kg) 
of material below the 0.05-oz/ton milling 
cutoff grade were stockpiled for future 
leaching. Pads were built in 1982 for 
several heap leaching tests involving ag- 
glomerated and unagglomerated ore, scale 
inhibitors, oxidizers, solution applica- 
tion techniques, etc. These pilot tests 
then formed the basis for designing a 
full-scale heap leaching facility that 
began operation in December 1982. Tests 
are now being conducted on material from 
the nearby Preble ore body. 

Other operations that have been dis- 
cussed by the mining press include the 
United Hearne Resources, Ltd. , operation 
near Hamilton, the Getchell property 
recently purchased by First Missis- 
sippi Corp. , the Bald Mountain property 



northeast of Ely, the Relief Canyon prop- 
erty being tested by Lacana Mining north- 
east of Reno, Dee Gold Mining Co.'s Boul- 
der Creek property near Carlin, Gila 
Mines Corp.'s mine near Reveille, the 
Borealis Mine managed by Houston Interna- 
tional Minerals Division of Tenneco, 
Inc. , the Tuscarora operation that be- 
longs to Tuscarora Associates, and the 
Gold Crown Mine in the Current Creek dis- 
trict operated by Great American Gold Co. 
The large number of leaching operations 
emerging in Nevada each year makes it im- 
possible to accurately track all of them. 
Since this is not the purpose of the 
report, other Nevada operations are not 
discussed. 

NEW MEXICO 

Solution mining operations have been 
relatively rare in New Mexico; only three 
have been identified. One of these is 
now inactive, while two are currently 
producing gold. All three are listed in 
the engineering data tables. 

Gold Fields Mining Corp. has brought 
the Ortiz Mine, 35 miles (56 km) south- 
east of Albuquerque, back into production 
with a heap leaching operation (22) . The 
activity stems from a mining and leaching 
plan submitted in 1978 to the New Mexico 
Health and Environment Department. Ore 
is being produced from a 3,000-ton/d (2.7 
X 10^ kg/d) open pit mine, which is be- 
ing dewatered by a series of peripheral 
wells. After the ore is crushed to less 
than 3/8 in (9.5 mm), it is placed on a 
pad with a traveling gantry system. Peb- 
ble lime is added to the ore to control 
pH yet minimize scale problems that are 
normally encountered when "milk-of-lime" 
is added to the makeup solution. Gold 
recovery has averaged 78 pet with a high 
of 87 pet since the operation opened. 

During the mid-1970' s interest revived 
in the old Cooney mining district around 
Mogollon. Mine dumps from the Confidence 
Mine and surface vein ore from the Eberle 
Mine were tested by Challenge Mining Co. 
for leachability with favorable results. 
The company has developed a 2-yr plan for 
heap leaching at a rate of 34,000 ton/yr 






29 



(31 X lOf' kg/yr) ( LS ) . After 2 yr this 
material will be depleted, which will re- 
quire underground mining in the old Eber- 
le workings. An estimated 300,000 tons 
(0.27 X 109 kg) grading 4 oz/ton (1.4 
g/kg) silver and 0.08 oz/ton (0.027 g/kg) 
gold remains in the mine. 

In 1975, Canorex opened a 500-ton/d 
(450 X 10^ kg/d) open pit gold mine to 
provide feed for a heap leaching opera- 
tion (_7 ) . Two adits in the ore body had 
proven ore reserves of 1 million tons 
(0.9 X 109 kg), grading 0.23 oz/ton (0.08 
g/kg) gold and 0.63 oz/ton (0.21 g/kg) 
silver. A plant was constructed to han- 
dle solution from the heaps. The opera- 
tion apparently went well for a while but 
has not produced recently. 

SOUTH DAKOTA 

Two heap leaching operations have 
been identified in South Dakota. One 



functioned at the pilot-scale level in 
1980; the other has applied for the per- 
mits necessary to construct a pilot 
facility. 

Cyprus Exploration is operating the 
pilot-scale leaching test at the Gilt 
Edge property 4 miles (6 km) southeast of 
Lead in the Black Hills. At the site of 
early mining activity, a pad and solution 
ponds were constructed for a 1,900-ton 
(1.7 X 10^ kg) heap. Further information 
is unavailable at this time. 

Tiaga Gold Corp. -Wharf Resources, Inc., 
has delineated a 5-million-ton (4.5 x 109 
kg) ore body at 0.04 oz/ton (0.014 g/kg) 
gold on their Anne Creek property 4 miles 
(6 km) southwest of Lead. Plans are be- 
ing made to heap-leach the ore with a 
system patterned after the Zortman- 
Landusky operations in Montana. 



PERMITTING REGULATIONS 



FEDERAL REGULATIONS 

The Resource Conservation and Recovery 
Act of 1976 (RCRA) has been the Federal 
regulatory action recently attracting the 
most attention from miners (34) . Aimed 
at improving resource conservation and 
recovery through land use control, the 
act establishes a Federal-State permit 
system for hazardous waste management. 
Originally the mining operators were con- 
sidered to be producers of hazardous 
waste and operators of hazardous waste 
treatment, storage, and disposal facili- 
ties. So many problems arose in attempt- 
ing to apply the regulations to mining 
operations, however, that an eimendment 
was passed in October 1980 to exempt min- 
ing from the law while studies were con- 
ducted. It is safe to assume, however, 
that this act will eventually impose some 
control. Since the material on a leach- 
ing pad and any leachate emanating from 
it could be considered hazardous wastes, 
leaching operations will fall under 
the purview of the act. Ground water 



monitoring programs will be required to 
assure that the uppermost aquifer will 
not be harmed. Although the U.S. Envi- 
ronmental Protection Agency (EPA) is 
charged with administering the permit and 
enforcement provisions of the act, EPA 
intends to pass on the permit system to 
the States. 

EPA is also charged with implementing 
the Safe Drinking Water Act and, partic- 
ularly interesting to in situ leaching 
operators, its Underground Injection Con- 
trol Program. This program establishes a 
permitting system for five classes of 
wells by which fluids are injected or 
disposed into the ground. EPA also regu- 
lates surface discharge permits under the 
National Pollutant Discharge Elimination 
System (NPDES) and the Prevention of Sig- 
nificant Deterioration Program, which may 
affect a potential leaching system. 
Operators should contact the closest re- 
gional EPA office for clarification on 
permits required (table 5) . 



30 



TABLE 5. - Environmental Protection Agency regional offices issuing hazardous 
waste and surface discharge permits 



Office 


Coverage 


Office 


Coverage 


Region IV 




Region IX 




Enforcement Division 


New Mexico 


Enforcement Division 


Arizona 


1st International Bldg. 


Texas 


215 Fremont St. 


California 


1201 Elm St. 




San Francisco, CA 94105 


Nevada 


Dallas, TX 75270 




(415) 556-2320 




(214) 749-1983 




Region X 




Region VIII 


Colorado 


Enforcement Division 


Idaho 


Enforcement Division 


Montana 


1200 6th Ave. 


Oregon 


Suite 900 


North Dakota 


Seattle, WA 98101 


Washington 


1860 Lincoln St, 


South Dakota 


(206) 442-1220 




Denver, Co 80203 


Utah 






(303) 837-3868 


Wyoming 







Leaching operations targeted for U.S. 
Forest Service, Bureau of Land Manage- 
ment, or other Federal lands will proba- 
bly require an environmental assessment 
before permission to proceed is granted. 
A prospective operator should clear this 
through the district office of the Fed- 
eral agency controlling the land. A 
helpful discussion of regulations and 
procedures is contained in "Forest Ser- 
vice Current Information Report No. 14," 
available at Forest Service offices. 

In addition, recent Bureau of Land Man- 
agement (BLM) regulations (43 CFR 3800) 
on Surface Management of Public Lands Un- 
der U.S. Mining Laws went into effect 
January 1, 1981, for the purpose of pre- 
venting undue degradation and requiring 
reasonable reclamation of BLM lands dis- 
turbed by any mining. Depending on the 
level of activity and size of the dis- 
turbance, a plan must be filed and some- 
times approved by BLM before mining can 
proceed. 

STATE REGULATIONS 

Permits required for leaching vary con- 
siderably from State to State. Table 6 
lists a few key permits and gives the as- 
sociated State agencies to be used as 
initial contacts. These contacts can 
provide details on which permits are re- 
quired and the regulations and require- 
ments pertaining to each. 



It is interesting to note that Califor- 
nia and Colorado have moved toward con- 
solidating permit requirements into one 
office. In California, the central con- 
tact is the Department of Economic and 
Business Development, which provides any 
new business venture with a complete list 
of necessary permits and directions for 
obtaining them. The county commissions 
assume leadership in California during 
the permitting process. 

Colorado has developed a voluntary pro- 
cess, tailored primarily for large opera- 
tions, whereby a company can request a 
Joint Review of a proposed mining proj- 
ect. If a company requests this Joint 
Review, the State coordinates interests 
of all Involved Federal, State, and local 
agencies and develops a statement of re- 
sponsibilities and problems that must 
be resolved and a schedule of activities 
during the regulatory time period. Com- 
panies that do not wish to use the Joint 
Review process may proceed as before, by 
interacting with the permitting agencies, 
a list of which can be obtained from the 
Colorado Department of Natural Resources 
(table 6). 

For the other States where leaching is 
practiced, table 6 lists the prime agency 
contacts. 



31 



TABLE 6. - State permitting agencies 



Agency 



Action 



Remarks 





ARIZONA 




Department of Natural Resources 


Booklet on laws and 




Mineral Bldg. , Fairgrounds 


regulations. 




Phoenix, AZ 85007 






(602) 255-3791 






State Office Bldg. 






415 West Congress, Room 190 






Tucson, AZ 85701 






(602) 882-5399 






State Mine Inspector 


Mining code and 


Must be notified of commencement or 


705 West Wing 


regulations. 


suspension of operation. Regulates 


Capitol Bldg. 




health and safety. 


Phoenix, AZ 85007 






(602) 255-5971 






Bureau of Water Quality Control 


Water discharge 




Arizona Department of Health 


permit. 




1740 West Adams 






Phoenix, AZ 85007 






(602) 255-1252 






Department of Water Resources 


New water supply 




99 East Virginia 


permit. 




Phoenix, AZ 85004 






(602) 255-1550 







CALIFORNIA 



Department of Economic and 


All necessary 


Provides all permitting information 


Business Development 


permits. 


for any business. 


1120 N St. 






Box 1499 






Sacramento, CA 95805 






(916) 322-1394 









COLORADO 




Department of Natural Resources 


Joint review 


The joint review process is an op- 


Executive Director's Office 


process. 


tional consolidated review proce- 


1313 Sherman St., Room 723 




dure for major energy and mineral 


Denver, CO 80203 




resource development projects. 


(303) 839-3337 








Colorado permit 


Operator may elect to obtain permit 




directory. 


directory and deal with permitting 
agencies. 





IDAHO 






Department of Lands 


Mining permit 






Bureau of Minerals 








State House 








Boise, ID 83720 








(608) 334-3569 








Manager Source Control Section 


Waste water treat- 


Review, 


no permit. 


Water Quality Bureau 


ment review. 






Division of Environment 








Deapartment of Health & Welfare 








State House 








Boise, ID 83720 








(208) 334-4059 









32 



TABLE 6. - State pennltting agencies — Continued 



Agency 



Actiou 



Remarks 





MONTANA 




Department of State Lands 


Operating permit... 


If over 36,000 ton/yr and/or 5 acres 


Reclamation Division 




disturbance; otherwise small mines' 


Hard Rock Bureau 




exclusion. 


Capitol Station 






Helena, MT 59601 






(406) 449-2074 






Department of Health and 


Water discharge 




Environmental Sciences 


permit. 




Water Quality Bureau 






Room 206 






Cogswell Bldg. 






Helena, MT 59601 






(406) 449-2406 







NEVADA 



Administrator 


Mining permit 


Also will provide a list of State 


Division of Mineral Resources 




and Federal permits required before 


Department of Conservation and 




a mining permit can be granted. 


Natural Resources 






Nye Bldg. 






201 South Fall St. 






Carson City, NV 89710 






(702) 885-4368 









NEW MEXICO 




Bureau Chief 


Ground water qual- 


Administer the Water Quality, Con- 


Health and Environment 


ity permit. 


trol Commissions regulations. 


Department 






Environmental Improvement 






Division 






Water Pollution Bureau 






Box 968 






Santa Fe, NM 87503 






(505) 827-5271 









SOUTH DAKOTA 




Land Management Specialist 


Reclamation permit. 


Lead agency; coordinates input from 


Department of Agriculture 




other agencies. 


Conservation Division 






Surface Mining Program 






Anderson Bldg. 






Pierre, SD 57514 






(605) 773-4201 






Department of Health 


Solid waste dispo- 




Division of Environment and 


sal permit. 




Health 






Joe Foss Bldg. 






Pierre, SD 57514 






(605) 773-3361 






Department of Water and 


Water quality 




Natural Resources 


permit. 




Division of Water Quality 






Joe Foss Bldg. 






Pierre, SD 57514 






(605) 773-3351 







33 



LEACHING PROBLEMS AND RESEARCH 



PERCOLATION 

Several problems hamper broader appli- 
cation of leaching methods for recovering 
gold and silver. A prime problem is the 
predominance of silt- and clay-size par- 
ticles in some ores that prevent uniform 
leaching solution percolation through 
the ore heaps. A high clay content will 
cause ponding and/or channeling, which 
blocks much of the ore from contact with 
the solution. Since small particles 
leach faster and more thoroughly than 
large ones, operators frequently crush 
the ore before leaching. Unfortunately 
crushing also can produce the extremely 
fine sizes that promote blockage and 
channeling. Even without crushing, some 
ores have a strong tendency to disinte- 
grate into clay through natural weather- 
ing agents and/or action of leaching 
solutions. Tests should be run on the 
ore to determine the percolation rate 
and metal recovery for various size 
distributions. 

Bureau of Mines research on agglomera- 
tion techniques provides the most promis- 
ing solution to clay problems; this was 
described in the section on Leaching 
Operations and in references 27 and 28. 
The several operators who have tried this 
technique have observed rather spectacu- 
lar inceases in percolation and recovery 
rates. Tombstone Exploration's Conten- 
tion Mine agglomerates its ore, as do the 
Alligator Ridge, Packard, and Candelaria 
operations. 

An extremely low permeability (tight) 
matrix presents a problem to leaching 
operators, again because the solution 
cannot reach the minerals. Weathered and 
oxidized ores generally leach better than 
unoxidized ones because oxidation breaks 
down an impeirmeable matrix. Heavier pow- 
der factors during mining may fracture 
the ore better, increasing the solution's 
access. A fresh, tight ore may, however, 
yield its values only if it is crushed 
and ground to a small enough size to ex- 
pose the metallic mineral grain. Crush- 
ing and in particular grinding, however, 



greatly boost the costs of preparing the 
ore for leaching. As previously men- 
tioned, it is imperative that laboratory 
tests be conducted to determine the most 
economical crushing size range. 

TEMPERATURE 

Solution temperature is a significant 
factor in leaching reactions. The chemi- 
cal reaction between gold and silver and 
cyanide proceeds much faster in warm so- 
lutions. Below 10° C (50° F) the reac- 
tion is appreciably slower than it is at 
normal summer operating temperatures. In 
practice, however, most operators work in 
cold weather until the solutions begin to 
freeze. Depending on its location, a 
mine can lose several months per year of 
potential leaching time in cold weather. 

A successful research program on meth- 
ods to heat solutions for leaching would 
provide a valuable improvement in the 
technology. A solar heating system would 
seem to be an ideal candidate since most 
leaching operations are located in arid 
regions with little cloud cover. Saline 
evaporation ponds may also be a possible 
source of heat for leaching solutions. 

The only reported research on solution 
heating is by Smoky Valley at its Round 
Mountain operation, where a 25-million- 
Btu/h (7.3-million-W) submersible kero- 
sine heater installed in the leach solu- 
tion pond has proven successful (26). 

SOLUTION LOSS 

At most leaching sites, 10 to 25 pet of 
the leaching solution is lost by evapora- 
tion. Since the leaching solution must 
contain high oxygen levels and since this 
is most easily achieved through spraying, 
evaporative losses seem inevitable. Im- 
portant research topics would be to de- 
termine this loss and to measure dis- 
solved oxygen in solutions as a function 
of the application method. 

Besides the evaporative losses, gangue 
minerals in the ore consume both the 



34 



cyanide and the lime or caustic soda. 
Consumption may run as high as 25 to 50 
pet. Since the Interactions between cya- 
nide solutions and most gangue minerals 
are well established, no new research is 
recommended for cyanide leaching. For 
those deposits that exhibit high cyanide 
consumption, research on alternative lix- 
iviants may be warranted. 



Principal efforts have been directed to- 
ward the subeconomic, lower grade ores 
that are unminable and/or untreatable 
using traditional methods. Although ma- 
jor gold and silver projects have been 
investigated by various Bureau Research 
Centers, most current activity is con- 
centrated at the Reno and Twin Cities 
Research Centers. 



CALCIUM SALT SCALE 

Calcium salts in distribution lines 
pose a major problem with most systems, 
particularly those that use lime for al- 
kalinity control. At least one operation 
also experienced problems with barium 
salts clogging spray nozzles. The miner- 
al scale prevents operators from using 
some small-orifice components in their 
distribution systems, such as the drip 
irrigation tubes used in copper leaching 
operations, and is particularly trouble- 
some in sprinklers; frequent soaking in 
hydrochloric acid is about the only cure. 
Several operators mix a scale inhibitor, 
such as the Surfo-H35 organic phosphate 
marketed by Baroid, with the leaching so- 
lution to keep the calcium minerals dis- 
solved. Some additives, however, occa- 
sionally form colloidal suspensions that 
plug filtration systems. 

Bagdad wigglers, first used to distri- 
bute solutions at the Bagdad copper mine, 
offer a possible means to combat calcium 
salt scale. Simple to construct (fig. 
7), the large-diameter tubing will remain 
open much longer than the small orifices 
of oscillating sprinklers and is easy to 
free from lime scale. 

Many operators have experimented with 
NaOH instead of lime to control pH with 
considerable reduction in the rate of 
lime scale. The relative costs must be 
carefully weighed, however, since NaOH is 
more expensive. 

RESEARCH ON NOVEL SOLUTION 
MINING METHODS 

The Bureau of Mines has actively inves- 
tigated new techniques in recovering 
and processing of gold and silver ores. 



In Situ Leaching 

In situ (in-place) leaching offers at- 
tractive benefits for producing metals at 
a minimum of capital investment and oper- 
ating cost, with low environmental degra- 
dation. The technique has not been ap- 
plied to gold and silver ores, primarily 
because of fear over public reaction to 
using cyanide solutions where ground wa- 
ter contamination is possible. Only one 
operation is known to have experimented 
with in-place gold and/or silver leach- 
ing, and that was with a noncyanide leach 
in an old underground mine, (See section 
on Colorado operations.) In this mine it 
was theorized that solutions could be in- 
jected into the fracture zone comprising 
the "vein" and that these solutions would 
migrate down the zone to drifts where 
they could be collected. To test the 
theory, water was injected into the vein 
and collected in a drift in a poured 
concrete trough (fig. 13). The experi- 
menters concluded that the trough (and 
drift) were not wide enough to encompass 
the vein-fracture system; excessive solu- 
tion losses occurred when the trough was 
bypassed. The experiment was never ex- 
panded to verify this conclusion because 
other metallurgical laboratory tests 
failed to resolve the chemistry of leach- 
ing that particular ore. 

Several companies have expressed a 
willingness to try leaching gold and 
silver in-place. Successful commercial 
leaching operations for uranium and cop- 
per have demonstrated that in-place 
leaching is economically feasible and 
environmentally safe. If a system can be 
developed for in-place-leaching gold and 
silver that will be publicly acceptable, 
small and/or low-grade deposits can be 
mined more cheaply and easily than 



35 



previously possible. The Bureau of Mines 
Twin Cities Research Center recently 
funded a contract study to evaluate the 
feasibility of such a system and to de- 
velop a conceptual design for It. The 
most likely system Involved blasting via 
vertical holes drilled from the surface 
to fragment shallow ore bodies , followed 
by solution application from the surface 
(36) . After the solution percolated down 
through the fragmented ore. It could be 
pumped to the surface from vertical re- 
covery wells or from underground solution 
collection drifts. Such a system would 
have a very good dls count ed-cash-f low re- 
turn on Investment with payout of less 
than 1 yr. 



The leached ore, which still contains 
the pregnant leaching solution. Is pushed 
by bulldozer or front-end loader to a 
sluice box where It Is slurried and 
pumped to a settling pond. After the 
spent ore has settled In the pond, the 
pregnant solution Is pumped from the pond 
to a recovery plant for removing the met- 
als. Meanwhile, the next layer of ore Is 
being cultivated as the cycle begins 
again. 

Although the field experiments to veri- 
fy this method were not conducted on gold 
and silver ores, laboratory experiments 
on them and field experiments on other 
ores worked well. 



Underground mines offer potential for 
In situ leaching. Ore would be blasted 
and left In stopes for leaching. After 
percolating through the rubbllzed ore, 
pregnant solution would be pumped to the 
surface for extraction of the metals. 

Placer deposits offer Intriguing possi- 
bilities for In-place leaching, and sev- 
eral years ago the Bureau conducted labo- 
ratory tests on cyanide placer deposits 
(33) . At some future date It Is reason- 
able to assume that placers could be 
leached by vertical Injection and recov- 
ery wells like those used for uranium In 
roll-front deposits. 

Leach Farming 

A patent granted In 1973 ( 23 ) describes 
a technique for leaching friable exposed 
ore bodies In place or conventionally 
mined ore that Is placed In extremely 
shallow heaps. With this technique the 
ore Is cultivated with standard farm 
equipment to loosen It to a depth of 6 to 
12 In (15 to 30 cm). Leaching solution 
Is added to the loosened ore, which Is 
periodically cultivated and kept moist 
for the next several days. This cultiva- 
tion assures uniform contact between 
leaching solution and the ore and pre- 
vents downward escape of the solution. 



Thin-Layer Leaching 

Thin-layer leaching, somewhat a variant 
of the leach farming method, was de- 
veloped for the copper mines of South 
America (^-A^) . It offers potential for 
leaching gold and silver ores , particu- 
larly If combined with the Bureau's ag- 
glomeration process. 

The first step In the method involves 
crushing the ore; minus 1/2 in (1.2 cm) 
was sufficient for the copper ores 
tested. Next, a concentrated leaching 
solution is mixed with the ore in a ro- 
tating drum until the ore contains 10 pet 
of solution. The ore is then placed in 
piles while the solution reacts with the 
ore. After 24 h the ore is transported 
to an Impervious pad, where it is spread 
out in layers approximately 3 ft (1 m) 
deep. The ore is then sprayed with di- 
lute leaching solution and/or water un- 
til the dissolved metals are rinsed out. 
This rinse solution is collected in ponds 
in the same manner as with any other heap 
leach and then pumped to a recovery plant 
for metals extraction. The damp tailing 
is then removed from the pad and piled in 
the disposal area. 



36 



SUMMARY 



Gold and silver leaching operations 
have blossomed in many mining districts 
of the Western United States, Most are 
associated with old "vein" mines where 
the elements were deposited in fractures 
near volcanic or other igneous activity. 
A few large operations have also been es- 
tablished on oxidized sedimentary depos- 
its containing very fine, disseminated 
gold. 

In surveying current operations, over 
80 were found that had conducted tests or 
were actively leaching on a commercial 
scale. Sufficient engineering data were 
obtained for 26 of these operations to 
justify their inclusion in tables that 
cover location, geology, heap configura- 
tion, influent solutions, effluent solu- 
tions, and extraction. 



One of the first problems encountered 
by a hopeful leaching operator is obtain- 
ing the necessary permits from Federal, 
State, and frequently local agencies. To 
provide a start, key Federal and State 
agencies are identified in tables 5 
and 6. 

The main problems hampering leaching 
operations are unfavorable mineralogy and 
poor solution percolation owing to high 
clay content. Cold temperature effects 
on gold and silver solubility and rapid 
lime buildup in the distribution system 
also severely hamper the operations. Re- 
cent and ongoing Bureau research and con- 
tinued work at various mining operations 
are helping improve the techniques. 



REFERENCES 



1. Ageton, R. W. , G. T. Krempasky, and 
W. L. Rice. A Systems Approach to Recov- 
ering Gold Resources in Jefferson County, 
Mont. Introductory Review. BuMines RI 
7305, 1969, 16 pp. 

2. Barsky, G, , S, J. Swainson, and 
N, Hedley. Dissolution of Gold and Sil- 
ver in Cyanide Solutions, Trans. AIME, 
V. 112, 1934, pp. 660-677. 

3. Brimm, E. 0. Thin-Layer Leaching 
of Copper Ores. Pres. at AIME Annu. 
Meeting, New Orleans, LA, Feb. 20, 1979, 
18 pp.; available from Holmes and Narver, 
Inc., 999 Town & Country Rd., Orange, CA 
92668, as publication HN 0953.02. 

4. Brimm, E. 0. , and P. H. Johnson. 
New TL Leaching Process. Pres, at Conf. 
of Metallurgists, Montreal, Quebec, Aug. 
28, 1978, 22 pp.; available from Holmes 
& Narver, Inc., 999 Town & Country Rd., 
Orange, CA 92668, as publication HN 
0953.00. 

5. Buttermann, S. C. Gold. BuMines 
Mineral Commodity Profile, 1978, 17 pp. 



6. California Mining Journal, Gold 
Reserve Corporation Reports Increasing 
Profits From Montana Mine. V. 51, No. 
12, 1982, p. 4, 

7. Chisolm, E. 0. Canadians Operat- 
ing Gold Leaching Operation in New Mexi- 
co. The Northern Mines, v. 61, No. 27, 
1975. pp. 59-60, 

8. Clevenger, G. H. , and M. H. Caron. 
The Treatment of Manganese-Silver Ores. 
BuMines Bull. 226, 1925, 110 pp. 

9. D'Andrea, D. V., P. G. Chamber- 
lain, and J. K. Ahlness. A Test Blast 
for In Situ Copper Leaching. Pres. at 
1978 AIME Annu. Meeting, Denver, CO, Feb. 
28-Mar. 2, 1978, SME Preprint 78-AS-112, 
6 pp. 

10. D'Andrea, D, V., W. C. Larson, 
L. R. Fletcher, P, G, Chamberlain, and 
W, H, Engelmann, In Situ Leaching Re- 
search in a Copper Deposit at the Emerald 
Isle Mine. BuMines RI 8236, 1977, 43 pp. 



37 



11. Denver Equipment Co. (Denver, CO). 
Cyanidation of Gold Ores. Bull. M3-87, 
1978, 8 pp. 

12. Dorr, J. V. N. , and F. L. Bosqui. 
Cyanidation and Concentration of Gold and 
Silver Ores. McGraw-Hill, 2d ed. , 1950, 
511 pp. 

13. Duncan, D. M. Open Pit Gold Min- 
ing at Cortez. Ch. in Colorado Mining 
Association 1974 Mining Yearbook, Colo- 
rado Mining Association, Denver, CO, 
1974, pp. 92-94, 

14. Duncan, D, M. , and T, J, Smolik, 
How Cortez Gold Mines Heap Leached Low 
Grade Ores at Two Nevada Properties. 
Eng, and Min. J., v. 178, No. 7, 1977, 
pp. 65-69. 

15. Engineering and Mining Journal. 
Heap Leaching Revives Montana Gold Dis- 
trict. V. 182, No. 1, 1981, pp. 90-91. 



16. 



Pelletizing Aids Tombstone 



Leaching Operation. V, 182, No, 1, 1981, 
pp, 94-95, 

17. Escapule, C. B, , L. W, Escapule, 
C. B, Escapule, and D. D. Rabb. Heap 
Leaching and Silver Recovery at the State 
of Maine. Ch. 19 in Interfacing Technol- 
ogies in Solution Mining, ed. by W. J, 
Schlitt and J, B, Hiskey (Proc, SME-SPE 
Int. Solution Mining Symp,) Soc, AIME, 
Littleton, CO, 1982, pp, 223-230. 

18, Eveleth, R, W. Eberle Mine: A 
Heap Leaching Case History, Min, Eng, , 
V. 31, No, 2, 1979, pp, 138-140, 

19. Hedley, N, , and T, Howard, Chem- 
istry of Cyanidation, American Cyanimide 
Co,, Wayne, NJ, 1968, 54 pp, 

20, Heinen, H, J, , D, G, Peterson, and 
R. E. Lindstrom. Gold Desorption From 
Activated Carbon With Alkaline Alcohol 
Solutions. Ch. 33 in World Mining and 
Metals Technology, ed. by A. Weiss 
(Proc, Joint Min, and Metall, Inst, of 
Japan-AIME, Meeting, Denver, CO, Sept, 
1-3, 1976). AIME, New York, 1976, 
pp. 551-564. 



21. Heinen, H. J., D. G. Peterson, and 
R. E. Lindstrom. Processing Gold Ores 
Using Heap Leach-Carbon Adsorption Meth- 
ods. BuMines IC 8770, 1978, 21 pp. 

22. Hickson, R, J. Heap Leaching 
Practices at Ortiz Gold Mine, Santa Fe 
County, New Mexico. Ch. 18 in Interfac- 
ing Technologies in Solution Mining, 
ed. by W. J. Schlitt and J, B, Hiskey 
(Proc, SME-SPE Int, Solution Mining 
Symp., Denver, CO, Nov. 18-20, 1981). 
Soc. Min. Eng. AIME, Littleton, CO, 
pp. 209-222. 

23. Lankenau, A. W, , and J, L. Lake 
(assigned to Hazen Research, Inc. , Gold- 
en, CO). Process for Heap Leaching Ores. 
U.S. Pat. 3,777,004, Dec. 4, 1973. 

24. Leaver, E. S. , J. A, Woolf , and 
N, K, Karchmer, Oxygen as an Aid in the 
Dissolution of Silver by Cyanide From 
Various Silver Minerals, BuMines RI 
3064, 1931, 15 pp, 

25. Lefler, C, A, Leaching Practices 
at Smoky Valley Mine. Ch, 6 in Gold and 
Silver Leaching, Recovery and Economics, 
ed, by W, J, Schlitt, W, C, Larson, and 
J, B, Hiskey (Proc. SME Sessions on So- 
lution Mining and Economics of Precious 
Metals, AIME Annu. Meeting, Chicago, IL, 
Feb. 22-26, 1981). Soc. Min. Eng. AIME, 
Littleton, CO, 1981, pp. 51-56. 

26. McClelland, G, E, , and J. A, 
Eisele, Improvements in Heap Leaching To 
Recover Silver and Gold From Low-Grade 
Resources, BuMines RI 8612, 1982, 26 pp. 

27. McClelland, G. E, , and S. D. Hill. 
Heap Leaching Gold-Silver Ores With Poor 
Percolation Characteristics. Ch, 5 in 
Gold and Silver Leaching, Recovery and 
Economics, ed, by W. J. Schlitt, W. C, 
Larson, and J, B, Hiskey (Proc, SME 
Sessions on Solution Mining and Economics 
of Previous Metals, AIME Annu, Meeting, 
Chicago, IL, Feb, 22-26, 1981), Soc. 
Min. Eng., Littleton, CO, 1981, pp. 43- 
50. 



38 



28. McClelland, G. E. , D. L. Pool, 
and J. A, Elsele. Agglomeration - Heap 
Leaching Operations in the Precious Met- 
als Industry. BuMines IC 8945, 1983, 16 
pp. 

29. Mining Record. Colorado Mining 
Activity Roundup 1978. Jan. 31, 1979, 
19 pp. 



30. 



Gold Ray Plans To Begin 



Leaching Operations by September. Aug. 
8, 1979, 1 p. 



Situ Leaching of Metallic Ores Other Than 
Copper and Uranium. Ch. 11 in Interfac- 
ing Technologies in Solution Mining, ed. 
by W. J. Schlitt and J. B. Hiskey 
(Proc. SME-SPE Int. Solution Mining 
Symp., Denver, CO, Nov. 18-20, 1981). 
Soc. Min. Eng. AIME, Littleton, CO, 1982, 
pp. 123-130. 

37. Potter, G. M. , and H. B. Salis- 
bury. Innovations in Gold Metallurgy. 
Min. Cong. J., v. 60, No. 7, 1974, 
pp. 54-57. 



31. 



Thunder Mountain Gold 



Makes Agreement To Explore Claims. Oct. 
25, 1978, 1 p. 

32. . Yellow Gold Signs Mineral 

Properties Agreement. Nov. 25, 1981, 
1 p. 

33. Nichols, I. L. , H. B. Salisbury, 
and B. K. Shibler. Laboratory Evalua- 
tion of Some Factors in Cyaniding Gold 
Placers. BuMines RI 7559, 1971, 12 pp. 

34. Nordhausen, E. A. The Status of 
Permitting Regulations as Affecting Hy- 
drometallurgy Industry. Pres. at Arizona 
Conference of AIME Annual Meeting, Tuc- 
son, AZ, Dec. 8, 1980; reprinted in 
Skillings' Min. Rev., v. 70, No. 4, 1981, 
pp. 10-12. 

35. Pizzaro, R. J., D. M. McBeth, and 
G. M. Potter. Heap Leaching Practice at 
the Carlin Gold Mining Co., Carlin, NV. 
Ch, 19 in Solution Mining Sjnnposixam, ed. 
by F. F. Apian, W. A. McKinney, and A. D. 
Pernichele (Mini Symp. SME-AIME Annu. 
Meeting, Dallas, TX, Feb. 25-27, 1974). 
AIME, 1974, pp. 253-267. 

36. Potter, G. M. , C. K. Chase, and 
P. G. Chamberlain. Feasibility of In 



38. Scheiner, B. J. , D. L. Pool, J. J. 
Sjoberg, and R. E. Lindstrom. Extraction 
of Silver From Refractory Ores. BuMines 
RI 7736, 1973, 11 pp. 

39. Skillings, D. N. , Jr. Pinson Min- 
ing Co. Marking First Full Year Produc- 
tion. Skillings' Min, Rev., v. 71, No. 
28, 1982, pp. 8-10. 

40. Smith, M. E. Pilot Scale Heap 
Leaching at the Pinson Mine, Humboldt 
County, Nevada. Pres. at SME-AIME Fall 
Meeting and Exhibit, Salt Lake City, UT, 
Oct. 19-21, 1983. SME Preprint 83-403, 
11 pp. 

41. White, L. Heap Leaching Will Pro- 
duce 85,000 Oz/Year of Dore' Bullion for 
Smoky Valley Mining. Eng. and Min. J. , 
V. 178, No. 7, 1977, pp. 70-72. 

42. Zadra, J. B. A Process for the 
Recovery of Gold From Activated Carbon by 
Leaching and Electrolysis. BuMines RI 
4672, 1950, 47 pp. 

43. Zadra, J. B., A. L. Engel, and 
H. J. Heinen. Process for Recovering 
Gold and Silver From Activated Carbon by 
Leaching and Electrolysis. BuMines RI 
4843, 1952, 32 pp. 



39 



APPENDIX. —GOLD AND SILVER LEACHING BIBLIOGRAPHY 



Addison, R, , and H. R. Prieto. Reagent 
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Bhappu, R. B. , M. F. Lewis, and J. A. 
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Caldon, F. (assigned to Sunshine Mining 
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Chamberlain, C, J. Newton, and 
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No. 10, 1969, pp. 90-91. 



40 



Chamberlain, P. G. , and M. G. Pojar. 
The Status of Gold and Silver Leaching 
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Economics, ed. by W, J, Schlitt, W, C. 
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of Precious Metals, AIME Anna. Meeting, 
Chicago, IL, Feb. 22-26, 1981). Soc. 
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Chamberlin, P. D. Heap Leaching and 
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Min. Cong. J., v. 67, No. 4, 1981, 
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Chase, C. K. Treatment of Manganifer- 
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View of Changed Precious Metal Economics, 
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ery and Economics, ed. by W, J. Schlitt, 
W. C. Larson, and J. B. Hiskey (Proc. 
SME Sessions on Solution Mining and Eco- 
nomics of Precious Metals, AIME Annu. 
Meeting, Chicago, IL, Feb. 22-26, 1981). 
Soc. Min. Eng. AIME, Littleton, CO, 1981, 
pp. 23-24. 

Chase, C. K. , W. S. Ransom, and D. L. 
Simpson. The Glitter Gets Better - the 
Two Year Record of Gold Heap Leaching at 
Smokey Valley Mining Div, - Copper Range 
Co., Round Mountain, Nevada. Pres. at 
109th Annu. Meeting, TMS , AIME, Las 
Vegas, NV, Feb. 24-28, 1980, preprint 
A80.3, 8 pp. 

Chen, T. P. (assigned to Cyprus Metal- 
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of Copper, Gold, Silver, Platinum, Zinc, 
Lead, Cadmium, Mercury, Tin, Cobalt, 
Nickel, Antimony, Bismuth or Other Metal. 
U.S. Pat. 3,930,969, Jan. 6, 1976. 

Chisholm, E, 0, Canadians Operating 
Gold Leaching Operation in New Mexico. 
The Northern Miner, v. 61, No. 27, 1975, 
pp. 59-60. 

Cho, E. H. , and C. H. Pitt. Adsorption 
of Silver Cyanide on Activated Charcoal. 
Metall. Trans, B, v. lOB, No, 2, 1979, 
pp, 159-164. 



Cho, E. H. , and C. H. Pitt. Kinetics 
and Thermodynamics of Silver Cyanide Ad- 
sorption on Activated Charcoal, Metall, 
Trans, B, , v, lOB, No, 2, 1979, pp. 165- 
169. 

Davidoff , C. Extraction of Silver From 
Silver Ore Cyanide Leach Solutions. U.S, 
Pat. 3,311,468, Mar. 28, 1967; British 
Pat. 1,079,615, Aug. 16, 1967, 

Deitz, G, A,, and J, Halpern, Reaction 
of Silver With Aqueous Solutions of Cya- 
nide and Oxygen, J, Metals, v. 5, No. 
9, 1953, pp. 1109-1116. 

de Lucio, F, , and M, Vargas. Silver 
Recovery Increased From 70 to 86 Percent. 
World Min., v. 32, No. 6, 1979, pp. 60- 
61, 

De Mull, T, J., and R, A, Womack. Heap 
Leaching Practice at Alligator Ridge, 
Pres, at SME-AIME Fall Meeting and Exhib- 
it, Salt Lake City, UT, Oct, 19-21, 1983. 
SME Preprint 83-403, 11 pp. 

Dietz, G. , Jr., and R. M, Skomoroskl 
(assigned to American Chemical and Refin- 
ing Co.), Aluminum Containing Precipita- 
tion Agent for Precious Metals and Method 
for Its Use. U.S. Pat. 4,092,154, May 
30, 1978, 

Dorr, J, V, N, , and F, L, Bosqui. Cya- 

nidation and Concentration of Gold and 

Silver Ores, McGraw-Hill, 2d ed, , 1950, 
511 pp, 

Douglas , H. The Economics of Silver 
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Leaching, Recovery and Economics, ed. by 
W. J, Schlitt, W. C, Larson, and J, B, 
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Mining and Economics of Precious Metals , 
AIME Annu, Meeting, Chicago, IL, Feb, 22- 
26, 1981), Soc, Min, Eng, AIME, Little- 
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Biiiunmi i um»»u«ii«iMiimJnm|Ul!llllHWtl|| 

,.^.uUuukMk.MbSBiUiiliil 



41 



Duncan, D, M. , and T. J. Smolik. How 
Cortez Gold Mines Heap-Leached Low Grade 
Gold Ores at Two Nevada Properties. Eng, 
and Min. J., v. 178, 1977, No. 7, pp. 65- 
69. 

Duncan, D. W. , C. C. Walden, and P. C. 
Trussell (assigned to British (Columbia 
Research Council) . Accelerated Method 
for Bacterial Leaching of Copper, Zinc, 
Nickel, Molybdenum, Gold, Silver, Cobalt, 
Cadmium, Tin, and Other Metal Values 
From Sulfide Ores Thereof. U.S. Pat. 
3,305,353, Feb. 21, 1967; Can. Pat. 
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Engel, A. L. , and H. J. Heinen. Leach- 
Precipitation-Flotation Tests of a Cali- 
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1957, 9 pp. 

Engineering and Mining Journal. Carlin 
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. High Gold Prices Spur Frantic 



Exploration. V. 181, No. 10, 1980, 
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. Remining Gold: Heap Leaching 



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. Silver and Gold: Heap Leaching 

Exploits Low Grade Feed. V. 176, No. 6, 
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,^ Two Joint Ventures Probe Mining 
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Escapule, C. B. , L. W. Escapule, C. B. 
Escapule, and D. D. Rabb. Heap Leaching 
and Silver Recovery at the State of 
Maine. Ch. 19 in Interfacing Technol- 
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Littleton, CO, Nov. 18-20, 1981, SME pre- 
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Eveleth, R. W. Eberle Mine: A Heap 
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The Treatment of Refractory 



Gold Ores Containing Carbonaceous Materi- 
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AIME Meeting, Chicago, IL, Feb. 22-26, 
1981). Soc. Min. Eng. AIME, Littleton, 
CO, 1981, pp. 17-22. 

Guay, W. J., and D. G. Peterson. Re- 
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42 



Hansen, S. M. , and D. F, Snoeberger 
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(Proc. Joint Min, and Metall. Inst, of 
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Heinen, H, J. , G. E. McClelland, and 
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. Enhancing Percolation Rates in 

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Littleton, CO, pp. 209-222. 

Hiskey, J. B. Thiourea as a Lixiviant 
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43 



Hussey, S. J., H, B. Salisbury, and 
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. Carbon-In-Pulp Gold Adsorption 

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J. B. Hlskey (Proc. SME Sessions on 
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Littleton, CO, 1981, pp. 51-56, 



44 



Lewis, F. M, , C. K. Chase, and R. B. 
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46 



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